Section I—Skeleton

Cranial Skeleton

Skull

As with other vertebrates, the skull (Figure 7.1) of the cat protects and supports the brain and sense organs and is used in food gathering and processing.

Examine a skull and identify its main features (Figure 7.2a-c). Anteriorly and ventrally, note the marginal series of teeth. Directly above the middle anterior teeth is a large opening, the nasal aperture, that leads into the nasal cavity. In life the opening is separated by a carti­laginous septum, so that two external nares or nostrils are present. On either side of the skull, dorsal to the posterior teeth, are the large orbits, the cavities that

(a) Skull, left lateral view

FIGURE 7.2 Skull of the cat in (a) left lateral, (b) ventral, and (c) dorsal views, with bones color-coded.

house the eyeballs. Posterior to each orbit is the expan­sive cranial wall. This surface is the temporal fossa. It protects the brain and serves as the origin for the tem­poral musculature, which helps to close the jaw. Phylo- genetically, the fossa was derived mainly from a depressed region of the skull; thus comes its designation as a fossa, even though in the cat, as in other mammals, it is convex.

The ventral margins of the orbit and fossa are marked by the laterally bowed zygomatic arch, which serves mainly for the origin of the masseteric muscular, another group of jaw-closing muscles. Just posterior to the pos­terior root of the zygomatic arch is the external audi­tory meatus, the opening and passageway leading to the middle and inner ear.

Examine the skull in ventral view. The flat region between the teeth is the hard palate. At its posterior end is the median internal opening of the nasal passage. In life there are two openings, the choanae. Further poste­riorly lie the paired, oval, and strongly convex tympanic bullae. The bulla forms the floor of the middle ear.

Nearly all the bones of the skull are paired. First iden­tify the paired bones along the dorsal midline. Anteri­orly, the small, triangular nasals form the dorsal margin of the external nares. Posterior to the nasals are the large frontals. These extensive bones contribute to the dorsal and lateral surfaces of the skull. Laterally they form most of the orbital walls. Note the postorbital process that projects ventrally from each frontal to demarcate the posterior margin of the orbit. Further posteriorly are the parietals. These large bones form most of the dorsal and lateral parts of the cranium. The small, interpari­etal bone is wedged between the posterior ends of the parietals. Usually in older individuals the interparietal fuses to the surrounding bones. The temporal lines are faint ridges that mark the boundary of muscular attach­ment on the temporal fossa. These lines curve postero- medially from the postorbital processes of the frontal. They continue posteriorly on the parietals, converging toward the sagittal crest, which lies mainly on the inter­parietal. The crest may be high and sharp or represented only by a rugose ridge.

The occipital bone forms the occiput, the skull’s poste­rior surface, and contributes to the basicranium (i.e., the base of the cranium). Its most notable features are the foramen magnum (Table 7.1 describes the functions of skull foramina) and the occipital condyles, which artic­ulate with the first vertebra and support the head on the neck. The occipital is formed from four separate bones.

Return to the front of the skull (see Figures 7.3 and 7.4). The lateral and ventral margins of the nasal aperture are formed by the premaxillae. Each is a fairly small bone, with a long, narrow vertical portion and a short ventral portion, into which are implanted the front teeth, or the upper incisors. There are three incisors in each pre­maxilla. Ventrally the premaxillae form the anterior part of the hard palate. Here, each premaxilla has medial and lateral branches separated by the palatine fissure.

The maxillae are larger bones that hold the rest of the teeth and, in ventral view, form most of the hard palate. The large, curved canine is the first tooth in the maxilla. The following three teeth are premolars. Phylogeneti- cally, they represent premolars 2-4 (P2-P4). Cats lost P1 during their evolution and a gap, the diastema, is present between the canine and the small, peglike P2. P3 is a larger, triangular tooth that is followed by the very long, bladelike P4. Note the small, final tooth (molar 1 or M1) set transversely and covered in lateral view by the posterior end of P4. The blade-like P4 is the main meat-shearing tooth, and is referred to as the car- nassial (Figure 7.5).

In lateral view, each maxilla makes a large contribution to the snout or rostrum, where it contacts the premax­illa, nasal, and frontal bones. It also contributes a small portion to the orbital margin, where it contacts the small, delicate lacrimal bone, which makes a small contribution to the anterior orbital wall. The conspicu­ous opening of the lacrimal canal lies at the anterior margin of the orbit. Finally, the maxilla passes ventral to the zygomatic bone and participates in the anterior root of the zygomatic arch. Note that the maxilla con­tributes a shelf that forms the anterior floor of the orbit. The infraorbital canal is the large passage, entirely within the maxilla, at the anterior end of the zygomatic arch.

The palatine bones complete the hard palate posteriorly. Ventrally each palatine is a horizontal plate of bone, and the internal opening of the nasal cavity lies posterior to these plates at the midline. The ventral opening of the small palatine canal lies very near the suture between palatine and maxilla, approximately medial to the ante­rior part of P3. Posteriorly each palatine forms a thin, vertical plate of bone. This portion helps to form the pterygoid blades or flanges, as well as the ventral part of the orbital wall (Figure 7.3). The region between the pterygoid blades forms the bony part of the tube-like nasopharynx (see page 182). The orbital portion of the

palatine (Figure 7.3) has two openings.

In ventral view, locate the posterior end of the palatine bones (Figure 7.4). They contact a complex, composite element, usually termed the sphenoid, consisting of various ossifications. The main part, or body, is the basisphenoid. The pterygoid processes of the basisphe- noid are the portions that contact the palatine and form the posterior end of the pterygoid blades. The narrow, hook-like posterior projection at the end of each ptery­goid blade is the hamulus, to which a palatal muscle is attached. The main part of the basisphenoid contributes to the basicranium. It lies ventrally between the anterior parts of the tympanic bullae. The lateral, wing-like portion of the sphenoid is the alisphenoid. It is exposed on the lateral surface of the skull, and will be discussed shortly. Return to the ventral surface and locate the pre­sphenoid, the elongated, narrow, median bone lying between the pterygoid blades. The presphenoid also has lateral extensions, the orbitosphenoids, that are exposed in the ventral orbital walls. The connection between the presphenoid and orbitosphenoid is concealed by the palatine. The orbitosphenoid contacts the palatine, frontal, and alisphenoid bones. A tiny ethmoid foramen (see Table 7.1) lies along the suture between the or- bitosphenoid and frontal, but it may be difficult to see.

FIGURE 7.3 Skull of the cat in left lateral view, with zygomatic arch removed.

A series of four openings into the cranial cavity lies just dorsal to the pterygoid blades, on the lateral surface of the skull (Figure 7.4). Locate these foramina. The two anterior foramina are approximately twice as large as the two posterior foramina. The most anterior foramen is the optic canal, and it lies entirely within the orbitosphenoid. The second is the orbital fissure.

Peer into the internal opening of the nasal cavity. Extending anteriorly from the presphenoid is the vomer. Its ventral surface is keeled. The vomer passes anteri­orly and contacts the premaxillae and the maxillae. Examine the nasal aperture to view the vomer in ante­rior view, and note its narrow trough-like form. The vomer contributes to the bony nasal septum, which helps partition the nasal cavity into left and right parts.

The nasal cavity is filled with delicate, scroll-like turbinate bones (Figure 7.5). Most of these scrolls are formed from the maxilloturbinate, a paired bone that is connected to the medial wall of each maxilla. The turbinates dorsally in the nasal cavity are formed from the ethmoturbinates, which form the bulk of the ethmoid bone. In some cats a tiny portion of the ethmoid is exposed on the orbital wall, just posterior to the lacrimal. Other parts of the ethmoid are the meseth- moid and the cribriform plate. The mesethmoid is a median bony plate that, together with the vomer, helps form the bony nasal septum. The cribriform plate is a transverse plate, pierced by many small foramina, that forms the anteroventral wall of the cranial cavity. The cribriform plate can be viewed in a sagitally sectioned skull (Figure 7.5) or one in which the skull roof has been removed.

The temporal bone consists of three components that are fused together: the squamous, petrous, and tym­panic portions. The squamosals are the large, flat bones ventral to the parietals that help complete the lateral wall of the braincase (Figure 7.3). Ventrally the squamosal sends out a projection, the zygomatic process, that extends laterally and then anteriorly to form the posterior part of the zygomatic arch. On the ventral surface of the lateral part of the zygomatic process is the mandibular fossa (Figure 7.4), a smooth, transverse groove that articulates with the lower jaw in forming the temporomandibular joint.

The petrous portion of the temporal includes the pet­rosal and mastoid bones. The petrosal contains the inner

FIGURE 7.4 Skull of the cat in ventral view.

ear and may be seen through the external auditory meatus; its medial part should be viewed in a sagittally sectioned skull (Figure 7.5). The tiny bones or ossicles of the middle ear are the malleus, incus, and stapes. The middle ear is essentially between the external auditory meatus and the petrosal and is covered ventrally by the tympanic bulla. The middle ear bones sometimes remain in place. The malleus is the slender, elongated bone lying across the inner end of the external auditory meatus. The other two are more difficult to distinguish, but the stapes lies medially and fits into the oval window of the petrosal.

When the middle ear bones are absent, the two lateral foramina of the petrosal can be identified by peering into the external auditory meatus. Move the skull so that you have an anterolateral view into the middle ear. The foramina lie posteriorly on the petrosal. The more ventral one is the round window and opens posteriorly. The oval window, mentioned above, lies dorsal to the round window and opens laterally.

The mastoid bone is the only part of the petrous portion that is exposed externally. It overlaps the posterolateral surface of the tympanic bulla. Its ventral portion forms the mastoid process. (It is tiny in the cat, but you can feel it protruding just behind your ear.) The small sty­lomastoid foramen opens just anterior to the mastoid process (Figures 7.3 and 7.4). The mastoid bone con­tinues dorsally on the occiput as a short wedge between the squamosal and occipital bones. The hyoid appara­tus articulates with the tympanic bulla just ventral to the mastoid process (Figure 7.3).

The tympanic portion of the temporal includes the rounded, oval tympanic bulla, which is formed from two ossifications. The ectotympanic mainly forms the ring of bone surrounding the external auditory

FIGURE 7.5 Right side of the skull of the cat shown in sagittal section.

meatus, but contributes to the bulla ventrolaterally. The rest of the bulla is formed by the endotympanic. Anteromedially the bulla sends a pointed wedge, the styloform process, onto the base of the skull. Lateral to the process is the large opening for the auditory tube (Figure 7.4).

Note the large jugular foramen in the base of the skull at the posteromedial margin of the tympanic bulla. It passes between the bulla, the basioccipital (medially), and the exoccipital (posteriorly). A second, much smaller opening lies on the posterior wall (i.e., the exoc- cipital) of the larger passage. This is the anterior opening of the hypoglossal canal. Identify the posterior opening by examining the medial wall of an occipital condyle (i.e., look into the foramen magnum) or a sagittal section (Figure 7.5). Pass a bristle through the hypoglossal canal to determine its course. Posterior and dorsal to the posterior opening of the hypoglossal canal is the posterior opening of the condyloid canal.

The zygomatic bone, contacting the maxilla anteriorly and the zygomatic process of the squamosal posteriorly, forms most of the zygomatic arch. It forms the ventral margin of the orbit, and it sends up a postorbital process that closely approaches the postorbital process of the frontal. Ventrally and laterally the zygomatic bears a crescentic, slightly concave region for origin of masse­teric musculature.

Examine a sagittal section of the skull (Figure 7.5). The posterior half of the skull contains the large cranial cavity for the brain. The cavity is subdivided into a small anterior cranial fossa that receives the olfactory bulbs of the brain (see Figure 7.40), a middle cranial fossa, the largest subdivision, which mainly houses the cere­brum, and a posterior cranial fossa. The last two fossae are separated by a partial bony septum, the tentorium. A depression, the sella turcica (shaped like a traditional Turkish saddle in humans, but not in the cat), of the basisphenoid bone on the floor of the middle cranial fossa houses the hypophysis (see Figure 7.40). Note the petrosal, which houses the inner ear. The large opening is the internal acoustic meatus for passage of the facial (CN VII) and vestibulocochlear (CN VIII) nerves (see Section VII).

Mandible

The mandible or lower jaw is formed on each side by a single bone, the dentary (Figure 7.6). Left and right den- taries articulate anteriorly at the mandibular symphysis. The horizontal part of the dentary, in which the teeth are implanted, is the body. Note the presence, normally, of three incisors, followed by a canine, which is sepa­rated from the three cheek teeth by a diastema. The cheek teeth include p3, p4, and the elongated, blade-like m1. The m1 is the carnassial of the lower jaw; hence the carnassial pair is P4/m1. Typically, two mental foramina (see below and Table 7.1 for function) are

FIGURE 7.6 Dentary of the cat in (a) lateral and (b) medial views.

present anteriorly on the lateral surface of the body (Figure 7.6a).

The part posterior to the body is the ascending ramus, which has three processes. The coronoid process is the largest, extending dorsally. The temporal muscle inserts mainly on its dorsolateral and medial surfaces. The mas­seteric fossa, the large, triangular depression on the lateral surface of the coronoid process, serves as part of the insertion area for the masseteric musculature. The transversely expanded condyloid process, forming the lower half of the temporomandibular joint, bears a semicylindrical facet for articulation with the mandib­ular fossa on the temporal bone. Manipulate the mandible on the skull, and note how the form of the joint permits mainly up and down (or orthal) move­ments of the jaw and restricts lateral motion. Identify the roughened area on the anteromedial surface of the condyloid process. It is the insertion site for the medial pterygoid muscle. The angular process is the projection of the posteroventral part of the dentary. Its medial surface serves as the insertion site for the lateral ptery­goid muscle and its lateral surface for part of the mas­seteric musculature.

A large mandibular foramen lies anterior and slightly dorsal to the angular process and the medial surface of the ramus (Figure 7.6b). This foramen is the posterior opening of the mandibular canal, which extends anteri­orly through the body for the passage of the blood vessels and the mandibular division of the trigeminal nerve. The mental foramina are the main anterior open­ings of the canal.

Hyoid Apparatus

The hyoid apparatus (Figures 7.1 and 7.7) is composed of several small bones, the phylogenetic remnants of some of the gill arches in fishes. It sits in the throat at the base of the tongue and supports the tongue and laryngeal muscles. It is composed of a median bar, the

basihyoid or body, which is oriented transversely at the anterior end of the larynx. There are two pairs of horns or cornua that extend from the basihyal. The lesser cornua are the longer, anterior pair, whereas the greater cornua are the smaller, posterior pair. Each lesser cornu consists of a chain of four ossicles that curves anteriorly and dorsally to attach to the skull. These elements are the ceratohyoid, epihyoid, stylohyoid, and tympanohy- oid, which has a ligamentous attachment to the tempo­ral bone of the skull. The greater cornu is formed on each side by the thyrohyoid, which articulates with thyroid cartilage of the larynx. The discrepancy between the size and names of the cornua derives from their con­dition in humans, in which the greater cornua are indeed larger, as the lesser horn is represented only by the ceratohyoid.

Key Terms: Cranial Skeleton

alisphenoid angular process anterior cranial fossa ascending ramus of

dentary

auditory tube

(Eustachian tube) ceratohyoid choanae condyloid canal condyloid process coronoid process cranial cavity cribriform plate dentary diastema ectotympanic endotympanic epihyoid ethmoid bone ethmoid foramen external auditory meatus external nares foramen magnum foramen ovale foramen rotundum frontal greater cornua hamulus hard palate hypoglossal canal incisors incus infraorbital canal

basicranium basioccipital basihyoid basisphenoid body, of dentary carnassial

(antorbital canal) internal acoustic meatus interparietal jugular foramen jugular process lacrimal bone lacrimal canal lesser cornua malleus mandible mandibular foramen mandibular fossa mandibular symphysis masseteric fossa mastoid bone

mastoid process maxilla (pl., maxillae) mental foramina mesethmoid middle cranial fossa nasal nasal aperture occipital bone occipital condyles optic canal orbital fissure orbitosphenoid orbits

oval window (vestibular window; fenestra vestibuli)

palatine bone palatine canal palatine fissure parietals

petrosal (periotic) posterior cranial fossa postorbital process premaxillae premolars presphenoid pterygoid blades rostrum

round window (fenestra cochlea)

sagittal crest sella turcica sphenoid sphenopalatine

foramen squamosal stapes styloform process stylohyoid stylomastoid foramen temporal bone temporal fossa temporal lines tentorium

thyrohyoid turbinate bones tympanic bullae tympanohyoid

vomer

zygomatic arch

zygomatic bone (malar, ^jug^al)

Postcranial Skeleton

Vertebral Column

The vertebral column is composed of a series of movable bones, or vertebrae. The column is an important struc­ture in support and locomotion and has evolved in mammals into five distinct regions. Examine a mounted skeleton to identify these regions (Figure 7.1). Most anteriorly, in the neck, are the cervical vertebrae, which are followed by the rib-bearing thoracic vertebrae in the trunk. Next are the lumbar vertebrae, lying dorsal to the abdomen and between the thoracics and sacral ver­tebrae. The latter are fused into a solid unit and articu­late with the pelvis. Lastly, the caudal vertebrae support the tail. An opening, the intervertebral foramen, is present on each side between adjacent vertebrae for the passage of a spinal nerve.

Each vertebra generally possesses several parts. For the following, examine a typical cervical vertebra (Figure 7.8). The centra (sing., centrum) or vertebral bodies, oval or kidney-shaped in section, form the main support of the vertebral column. The neural canal, through which the spinal cord passes, lies dorsal to the centrum. The neural canal is enclosed laterally and dorsally by the neural arch. The arch is composed on each side by a pedicle, an oblique plate forming the side of the arch, and a lamina, a horizontal plate forming the top of the arch. A neural process projects dorsally from the neural arch. A transverse process projects laterally on either side from near the junction of the centrum with a pedicle. Near the junction between the pedicle and lamina, each vertebra has anterior and posterior pro­jections for articulation with other vertebrae. The ante­rior projection, the prezygapophysis, bears a facet that faces dorsally or dorsomedially. The facet of the poste­rior process or the postzygapophysis faces ventrally or ventromedially. Thus, a prezygapophysis slips under the postzygapophysis of the preceding vertebrae.

These features are common to most vertebrae, though they may vary in size and proportion among vertebral types. Some vertebrae may also have additional pro­cesses. Examine a set of disarticulated vertebrae and distinguish the main types, as described below.

Cervical Vertebrae

The cat has seven cervical vertebrae, as do most mammals. The first two are specialized and markedly dif­ferent from the remaining five, due to their function in support and movement of the head. The first vertebra is

FIGURE 7.8 Cervical vertebra of the cat.

FIGURE 7.9 Atlas of the cat in posterolateral and ventral views.

the atlas (Figure 7.9), after the mythological giant Atlas, who was charged with supporting the world on his shoul­ders. The atlas is easily recognizable. It is ring-like and lacks a neural process. Ventrally, the centrum is replaced by a narrow strut, and the neural arch is the more promi­nent component. The transverse processes are broad, wing-like structures. The prezygapophyses bear strongly curved articular surfaces for the occipital condyles. The joints between the occipital condyles and atlas permit mainly up and down or nodding movements of the head on the neck. The postzygapophyses bear nearly flattened surfaces for articulation with the second cervical verte­bra, the axis. These joints allow mainly rotational move­ments of the head. The paired atlantal foramina pierce the vertebral arch transversely just dorsal to the prezy- gapophyses. Vessels (such as the craniodorsal artery, a branch of the vertebral artery) and the first cervical nerve pass through the foramen. The transverse foramina, despite their name, do not run transversely but antero- posteriorly through the base of the transverse process. The transverse foramen, through which the vertebral artery passes, is characteristic of the cervical vertebrae, except for the last, in which it is usually absent.

The axis is also distinctive (Figure 7.10). It has a large and anteroposteriorly elongated neural process that hangs over the atlas. Also, it has a dens, a process that pro­jects anteriorly between the prezygapophyses and articu­lates with the atlas. This articulation participates in shaking movements of the head. Note the slender, poste­riorly projecting transverse processes. The remaining cer­vical vertebrae are more typical vertebrae (Figure 7.8). Each possesses a relatively slender neural process that increases in height from the third cervical, in which the process is very strongly reduced, to the seventh cervical.

Thoracic Vertebrae

The cat usually has 13 thoracic vertebrae. These are distinguishable by the presence of additional facets for articulation with ribs and very short transverse processes. The capitulum of a rib (see below) sits between and articulates with two successive centra, for which most thoracic vertebrae possess costal demifacets (Figure 7.11). These are anteriorly and posteriorly paired facets, so that a vertebra typically has four demi­facets. The demifacets of successive vertebrae form (on each side) a complete or “full” articular surface for the capitulum of a rib. Thoracics also have a costal facet on the ventral surface of each transverse process for artic­ulation with the tuberculum of the corresponding rib.

The thoracic series undergoes various changes as it pro­ceeds posteriorly. For example, the first thoracic has paired costal facets anteriorly and paired demifacets posteriorly. The last few thoracics, usually the 11th

(a) Axis, left lateral view

FIGURE 7.10 Axis of the cat in left lateral and ventral views.

through 13th, only have paired costal facets on the centrum (i.e., no demifacets). Also, the 11th thoracic has markedly reduced transverse processes, while the 12th and 13th lack transverse processes (reflecting the absence of a tuberculum on the last few ribs). The last few thoracic vertebrae have a slender accessory process (see Figure 7.12) that extends posteriorly from the pedicle, just lateral to each postzygapophysis. The acces­sory processes reinforce the articulation between the pre- and postzygapophyses.

Finally, note the change in the neural process through­out the thoracic series. Anteriorly, the process is high and inclines posteriorly. The height of the process tends to decrease posteriorly while inclination tends to increase until, depending on the individual, the 10th or 11th thoracic vertebra, at which point the process is markedly reduced and points anteriorly. This vertebra is known as the anticlinal vertebra (Figure 7.1), denot­ing the change in inclination. The neural process of the remaining thoracics (as well as the lumbar vertebrae) also incline anteriorly and increase in height. The change inclination of the neural processes allows the cat to extend and flex its back, especially during running.

Lumbar Vertebrae

The cat typically has seven lumbar vertebrae. They are characterized by their large size, which increases posteri­orly through the lumbar series, and pleurapophyses, elongated, blade-like processes that sweep anteroven- trally (Figure 7.12). A pleurapophysis represents a fusion of a transverse process and an embryonic rib. The prezy- gapophyses are extended dorsally beyond the articular surfaces into mammillary processes for attachment of epaxial muscles. Finally, note that accessory processes are present on all but the last two lumbar vertebrae.

Sacral Vertebrae

The cat has three sacral vertebrae that are fused together to form a single structure, the sacrum, which links the

FIGURE 7.11 Thoracic vertebra of the cat in left lateral and anterior views.

FIGURE 7.12 Lumbar vertebra of the cat in left lateral and ventral views.

FIGURE 7.13 Sacrum of the cat in dorsal and ventral views.

spine to the pelvic girdle (Figure 7.13). Anteriorly the sacrum has on either side a broad, rugose, and nearly circular surface for articulation with the innominate bones of the pelvis. The sacral vertebrae decrease in size posteriorly. Even though they are fused together, many of the structures can be recognized. Identify the three neural processes, and note the areas of fusion at the pre- and postzygapophyses. The pleurapophyses of the sacrals are expanded anteriorly and posteriorly and fuse with each other to enclose two pairs of sacral foramina on the dorsal and ventral surfaces of the sacrum for the passage of dorsal branches of spinal nerves. The pleu- rapophysis of the first sacral forms most of the articu­lar surface for the pelvis.

Caudal Vertebrae

Cats typically possess from 21 to 23 caudal vertebrae, which are the smallest of the vertebrae (Figure 7.14). The caudals tend to become progressively smaller and less complex posteriorly. The anterior caudals have zygapophyses and neural and transverse processes, but the more posterior vertebrae are elongated, cylindrical structures consisting almost entirely of the centrum. Beginning with the third or fourth caudal vertebra, you might note the presence of hemal arches, small V-shaped bones, lying at the anterior end of the ventral surface of the vertebra. Successive hemal arches enclose the hemal canal, through which caudal blood vessels pass. Each

FIGURE 7.14 Caudal vertebra of the cat in ventral view.

hemal arch articulates with a pair of small tubercles, the hemal processes, on the centrum. The small, delicate arches are usually lost during preparation, but their position can be determined by identifying the hemal processes.

Ribs

The ribs form a strong but partly flexible cage that pro­tects vital organs (e.g., heart and lungs) and participates in inspiration and expiration. Each rib articulates dor- sally with the vertebral column and ventrally with a costal cartilage.

The 13 pairs of ribs (Figure 7.15) may be subdivided into three types: vertebrosternal, vertebrochondral, and vertebral, based on the attachment of their costal carti­lages onto the sternum or breast bone (see Figure 7.16). The first nine ribs are vertebrosternal (or “true” ribs), meaning that the costal cartilages attach directly to the sternum. The next three are vertebrochondral (or “false” ribs) because their costal cartilages attach to the costal cartilage of another rib; that is, they do not attach directly to the sternum. The last rib is a vertebral rib because its short cartilage does not gain access to the sternum. It is thus also known as a floating rib.

Ribs differ in length but are generally similar in being curved, slender, rod-like bones. The capitulum or head bears surfaces for articulation with the demifacets of the thoracic vertebrae. This is followed by a short, con­stricted neck and then by the tuberculum, which has a facet for articulation with the costal facet on the trans­verse process of a thoracic rib. A tuberculum is absent in the final two or three ribs. The shaft of the rib curves distally, and bears a well-defined angle beyond the tuberculum. A costal cartilage is attached to the distal surface of each rib.

Sternum

The sternum (Figure 7.16) consists of eight sternebrae arranged anteroposteriorly midventrally on the thorax. The most anterior of the series is the spear tip-shaped manubrium. In some individuals the manubrium appears to be formed by the fusion of two elements. The next six sternebrae, constituting the body of the sternum, tend to be elongated, spindle-shaped elements. Finally, there is the elongated and tapering xiphister- num. The xiphoid cartilage attaches to its posterior end. The costal cartilages of the vertebrosternal ribs attach directly to the sternum. Typically, the pattern is that a costal cartilage attaches between adjacent sternebrae.

Forelimb

Scapula

The scapula or shoulder blade (Figures 17a, b) is a flat, triangular bone. Its medial surface is nearly flat, whereas its lateral surface has a prominent scapular spine. Examine a mounted skeleton, and note that the apex of the scapula is directed ventrally. Identify the scapula’s anterior, dorsal, and posterior borders. The glenoid fossa is the smooth, concave surface at the apex for articulation with the humerus. The delicate coracoid process projects medially from the anterior margin of the glenoid fossa and is the site of origin for the cora­cobrachialis muscle.

The medial surface bears the subscapular fossa. It is rel­atively flat, with a few prominent scar ridges indicating tendinous muscular insertions. Note the prominent ridge near the posterior border that demarcates a

FIGURE 7.16 Sternum and costal cartilages of the cat in ventral view.

narrow, slightly concave surface for muscular attach­ment. On the lateral surface, the scapular spine rises prominently and separates the supraspinous fossa anteriorly from the infraspinous fossa posteriorly, both of which are fairly smooth surfaces. Ventrally the spine ends in the acromion process. Just dorsal to the acromion process is the posteriorly projecting meta- cromion process.

Clavicle

The clavicle is a small, slender, slightly curved bone sus­pended in the musculature associated with the forelimb (Figures 7.1 and 7.17c). It lies just anterior to the prox­imal end of the humerus (see below). As such, the clav­icle of the cat does not articulate with other skeletal elements.

Humerus

The humerus (Figure 7.18) is the bone of the brachium, or proximal part of the forelimb. It articulates proxi­mally with the glenoid fossa of the scapula and distally with the radius and ulna. The head is the large, smooth, and rounded surface that articulates with the glenoid fossa of the scapula. It is best seen in proximal and posterior views. Lateral to the head is the greater tuberosity, whereas the lesser tuberosity lies medially. Anteriorly between the tuberosities is the deep bicipital groove, along which passes the tendon of the biceps brachii muscle. The pectoral ridge extends distally from the greater tuberosity on the anterior surface of the humeral shaft. The less prominent deltoid ridge ex­tends distally and obliquely from the posterior part of the greater tuberosity. This ridge converges toward and meets the pectoral ridge about midway along the shaft.

Distally, there is a smooth, spool-shaped surface, the condyle, which is actually composed of two articular surfaces. The smaller, lateral surface, the capitulum, articulates with the radius (see below), while the larger, medial surface, the trochlea, articulates with the ulna (see below). The coronoid and radial fossae are shallow depressions that lie just proximal to the trochlea and capitulum, respectively, on the anterior surface of the shaft. On the posterior surface, just proximal to the condyle, is the deep olecranon fossa that receives the olecranon of the ulna. The medial and lateral epi­condyles are the rugose prominences that lie on either

FIGURE 7.17 Right scapula of the cat in (a) right lateral and (b) medial views; and (c) clavicles of the cat.

side of the trochlea. The Supracondyloid foramen is the oval passage lying proximal to the trochlea.

Ulna

The ulna (Figure 7.19) is the longer of the two bones in the forearm, or antebrachium. It has a prominent prox­imal portion, but its shaft tapers distally. Proximally the ulna articulates with the humerus and the radius. Dis­tally it contacts the carpus or wrist. The trochlear notch is the deep semicircular surface for articulation with the trochlea of the humerus. The olecranon is the squared process extending proximal to the trochlear notch. It is the site of insertion of the tendon of the triceps brachii muscle. The coronoid proceSS extends anteriorly from the distal base of the trochlear notch. The radial notch, for articulation with the head of the radius, is a curved surface that lies medially along the base of the trochlear notch. The anterior part of the radial notch forms the lateral surface of the coronoid process. The roughened interoSSeuS creSt lies on the central portion of the medial surface of the ulnar shaft. A sheet of connective tissue extends between the ulnar interosseus crest and the radial interosseus crest (see below) and helps stabilize the antebrachium. The ulna tapers distally and ends in the Styloid proceSS, which articulates with the lateral part of the carpus.

Radius

The radius (Figure 7.19) is the second bone of the ante­brachium. In contrast to the ulna, the radius is more slender proximally and widens distally. As the radius articulates with the lateral part of the humerus and the medial part of the carpus, it crosses over the ulna when a cat assumes a normal standing position (i.e., the manus is pronated, meaning the palm faces the ground; the opposite position, with the palm directed up, is termed supinated). Proximally the radius consists of the head, which bears an oval, concave fovea that articu­lates with the capitulum of the humerus. The shape of the head allows the radius to rotate on the capitulum. Immediately distal to the fovea, the head bears a smooth, narrow strip, the articular circumference, that articulates with the radial notch of the ulna. The neck of the radius is a short segment between the head and the bicipital tuberosity, onto which the tendon for the biceps brachii muscle inserts. The roughened interosseus crest lies distally from the tuberosity on the medial edge of the shaft.

The distal end of the radius has a small articular surface for the ulna medially that allows the radius to turn on the ulna. The distal surface has a large, concave surface that articulates with the scapholunar bone of the wrist.

FIGURE 7.18 Left humerus of the cat in anterior and posterior views.

Note the two grooves on the anterior surface for passage of extensor tendons, and the short styloid process pro­jecting distally from the medial surface of the radius.

Manus

The manus (Figure 7.20) includes three portions: the carpus (wrist), metacarpus, and phalanges in proximal to distal order. The cat has five digits (fingers), each of which is formed by a metacarpal and phalanges. The carpus consists of seven small, irregularly shaped bones set in two rows. In medial to lateral order, the proximal row consists of the scapholunar and cuneiform, with the pisiform projecting from the ventral surface of the cuneiform (and thus not visible in Figure 7.20); the distal row consists of the trapezium, trapezoid, magnum, and unciform. These bones articulate distally with the metacarpals. Note how the bones of the carpus and the metacarpals are arranged in an interlocking pattern that restricts motion.

The metacarpals are included in the palm of the manus. Of the five metacarpals, metacarpal 1 is the most medial and shortest, being less than half the length of metacarpals 2-5. Each metacarpal consists of a base proximally, a shaft, and a head distally. Digit 1 has only a proximal phalanx, followed by the ungual phalanx, which bears a large claw. Digits 2-5 each have three seg­ments, the proximal, intermediate, and ungual pha­langes; the latter also bear large claws.

Hind Limb

Pelvis

The pelvis or hip consists of paired innominate bones (Figure 7.21) that articulate with each other ventrally at the pelvic symphysis and with the sacrum dorsally. Examine a mounted specimen, and note the position and orientation of the innominate. Each innominate is composed largely from three bones, the ilium, ischium, and pubis, although a fourth center of ossification, the acetabular bone, makes a small contribution. These bones are firmly fused together in the adult. Examine an innominate bone in lateral view and note the large ventral opening, the obturator foramen. Locate the acetabulum, the deep socket that receives the head of the femur (see below).

The ilium consists of a body, near the acetabulum, and an anterodorsally projecting wing. The iliac crest is the roughened, anterodorsal edge of the ilium. The rugose articular surface for the sacrum lies on the medial surface of the wing. The ischium extends posteriorly from the acetabulum and has an expanded termination, the ischial tuberosity. The pubis and the rest of the ischium are oriented ventromedially. Both bones con­tribute to the medial margin of the obturator foramen. Also, the ischium and pubis of each side of the body meet to form, respectively, the ischial and pubic sym­physes, which together form the pelvic symphysis. The acetabular bone forms the thin, medial part of the acetabulum.

Femur

The femur (Figure 7.22) is the bone of the proximal part of the hind limb, or thigh. The head of the femur is a hemi­spherical surface that fits into the acetabulum of the innominate. The head is supported by the neck, which projects obliquely from the proximal end of the femur. Lateral to the head is the roughened, proximally project­ing greater trochanter, which serves for attachments of hip musculature. The deep depression posteriorly between the trochanter and head is the trochanteric fossa. On the posterior surface of the shaft, just distal to the head, is the lesser trochanter. The intertrochanteric line connects the two trochanters, but is especially defined along the posterior edge of the greater trochanter. The most prominent muscular insertion site on the diaphysis is the linea aspera, the roughened ridge that extends diag­onally along the posterior surface of the femur.

The femur expands distally into the prominent and pos­teriorly projecting lateral and medial condyles. Each

FIGURE 7.19 Bones of the left antebrachium of the cat; (a) radius and ulna in anterior view, (b) ulna in lateral view, and (c) ulna and radius in posterior view.

condyle bears a smooth, semicircular surface for artic­ulation with the tibia. The intercondyloid fossa is the depression posteriorly between the condyles. The rugose areas for muscular attachment proximal to the condyles are the lateral and medial epicondyles. The patellar trochlea, for articulation with the patella, lies anteriorly between the condyles. It is a smooth, shallow trough, oriented proximodistally.

Patella

The patella (Figure 7.22), or kneecap, is a small, tear­shaped sesamoid bone, with its apex directed distally. Its anterior surface is roughened. Posteriorly it bears a smooth, shallowly concave surface for articulation with the femur.

Tibia

The tibia (Figure 7.23) is the larger and medial bone of the crus, or middle segment of the hindlimb. Its proxi­mal surface bears lateral and medial condyles that artic­ulate with the femur. Just distal to the lateral condyle, on the lateral surface and facing distally, is the small, nearly oval facet for the head of the fibula (see below). On the posterior surface of the tibia, between the condyles, is the popliteal notch. A small muscle, the popliteus, lies in the notch and is a flexor of the knee joint. The tibial tuberosity, for insertion of the patellar ligament, lies anteriorly. The tibial crest continues dis­tally from the tuberosity along the shaft.

The distal end of the tibia has two articular surfaces. The large surface on the distal surface, the cochlea tibiae, is for the astragalus, the tarsal bone with which the pes articulates with the hindlimb. Note how this facet consists of two sulci separated by a median ridge. This structure restricts motion at the ankle almost entirely to a fore and aft direction, producing flexion and extension. If available, manipulate the tibia and pes to observe this. The small, nearly triangular fibular facet, for articulation with the fibula, faces posterolat- erally and is contiguous with the lateral part of the cochlea tibae. The medial malleolus is the distal exten­sion of the tibia’s medial surface. It forms the medial protrusion of the ankle.

Fibula

The fibula (Figure 7.23) is the slender, lateral, and shorter bone of the crus. The head is irregular and expanded. It bears a proximal facet for articulation with

Figure 7.20 Right manus of the cat in dorsal view.

the tibia. The slender shaft widens distally. There are two distal facets, both toward the anterior half of the medial surface. The more proximal facet is for the distal articulation with the tibia. The distal facet articulates with the lateral part of the trochlea of the astragalus. The lateral malleolus projects distally from the postero­lateral end of the fibula.

Pes

The pes (Figure 7.24) consists of tarsals, metatarsals, and phalanges. There are seven tarsals, but the two most proximal bones, the astragalus and calcaneum, are much larger than the others. The astragalus is the medial bone. It articulates proximally with the tibia and fibula. Note the form of the surface, or trochlea tali, for artic­ulation with the tibia. The trochlea consists of medial and lateral keeled surfaces separated by a sulcus. The astragalus articulates ventrally with the calcaneum, which lies laterally. It is about twice as long as the astra­galus and projects posteriorly as the heel. Distally the astragalus articulates with the navicular, while the cal­caneum articulates with the cuboid. The navicular artic­ulates distally with the lateral, intermediate, and medial cuneiform bones, and laterally with the cuboid. Note how the articulations among the tarsals and metatarsals are arranged to produce interlocking joints that tend to restrict movement. For example, the lateral cuneiform articulates with metatarsal III distally, but its medial surface articulates with both the intermediate cuneiform and metatarsal II. There are five metatarsals. The first is strongly reduced to a small nub that articulates with the medial cuneiform. The phalanges for digit 1 have been lost in the cat. The remaining metatarsals are stout,

FIGURE 7.21 Left os coxae of the cat in (a) lateral, (b) ventrolateral, and (c) medial views.

FIGURE 7.22 Left femur and patella of the cat in (a) anterior and (b) posterior views.

FIGURE 7.23 Left tibia and fibula of the cat in (a) anterior and (b) posterior views.

elongated elements and each articulates with a series of three phalanges, the proximal, intermediate, and ungual phalanges.

Key Terms: Postcranial Skeleton

accessory process acetabular bone acetabulum acromion process angle

antebrachium (forearm) anterior, dorsal, posterior borders of scapula anticlinal vertebra articular circumference astragalus (talus) atlantal foramen

(plur., foramina; intervertebral foramen)

atlas

axis

bicipital groove bicipital tuberosity body

calcaneum (calcaneus) capitulum (head) of rib carpus

caudal vertebrae

centra (sing., centrum) cervical vertebrae clavicle

cochlea tibiae

condyle coracoid process coronoid fossa

coronoid process costal cartilage costal demifacet costal facet cuboid

cuneiform (lateral,

medial, intermediate) deltoid ridge dens

digits

femur

fibula

fibular facet

fovea

Figure 7.24 Right pes of the cat in dorsal view.

glenoid fossa greater trochanter greater tuberosity head of femur head of humerus head of radius hemal arches hemal processes humerus iliac crest ilium infraspinous fossa innominate bones intercondyloid fossa interosseus crest of radius

interosseus crest of ulna intervertebral foramen ischial symphysis ischial tuberosity ischium

lamina

lateral condyle lateral epicondyles lateral malleolus lesser trochanter lesser tuberosity linea aspera lumbar vertebrae magnum mammillary processes manubrium medial condyles medial epicondyle medial malleolus metacarpals metacarpus metacromion process metatarsals navicular neck of femur neck of radius

neck of rib neural arch neural canal (vertebral

canal)

neural process (spinous process)

obturator foramen olecranon olecranon fossa patella patellar trochlea pectoral ridge pedicle pelvic symphysis phalanges pisiform pleurapophyses popliteal notch postzygapophysis prezygapophysis pubic symphysis pubis (pl., pubes) radial fossa radial notch radius rib sacral foramina sacral vertebrae scapholunar scapula scapular spine shaft of humerus

(diaphysis)

shaft of femur

(diaphysis)