Section III—Muscular System
Study of the musculature requires that the skin be removed. There are various techniques that allow efficient skinning. One of these is that a scalpel, in most cases, be used as little as possible.
Remember that dissection does not have as its goal the frequent use of a scalpel, as many students would like to believe. Instead, one may recognize skill in dissection by selection of the proper implement for the task at hand. The scalpel has limited application. It may be used to cut the skin and then to help reflect it. In the latter procedure, the scalpel should be used in a sideways scraping motion. Turning the blade axially 180° and using its point as a pick can also be very effective. Its cutting edge should rarely be used in separating skin from underlying body structures.Trunk and Appendicular Muscles
The trunk musculature is considered first for various reasons: It is relatively easy to examine as an introduction to the musculature; it comprises the bulk of the musculature of the body; and the procedure will give you practice in skinning a region where mistakes and inexperience will not produce serious damage.
Peel back a strip of skin, about 10 cm in width, from one side of the trunk region just posterior to the anterior dorsal fin (Figure 3.16, flap 1). Cut with a scalpel three sides of a rectangular section of skin. Begin the incisions middorsally and continue ventrally down the body to the midventral plane. The skin is thin, so cut carefully to avoid damaging underlying structures. Do not cut the fourth side. This will produce a flap of skin that can be wrapped around exposed tissues at the end of a lab period to help protect them. After cutting the edges of the flap, lift one corner and begin to pull the skin away from the trunk musculature. As the skin begins to pull away, scrape the connective tissue between skin and muscles.
Use the non-cutting edge of your scalpel, as described above. Skin will come off easily in some spots but will stick in others because muscle fibers attach directly to the dermis. Patiently scrape the muscle fibers from the inside of the skin toward the body with a scalpel. In some areas, a probe or the ends of narrow forceps will suffice.
FIGURE 3.15 Enlarged views of the surface of the skin of the shark. a. Ampullae of Lorenzini. b. Endolymphatic pores.
Once flap 1 is reflected, notice that the trunk musculature is divided into the dorsal epaxial musculature and the ventral hypaxial musculature by the horizontal skeletogenous septum, a connective sheet lying in the frontal plane (Figure 3.17). Each of these primary divisions consists of longitudinally segmented, Z-shaped myomeres, separated by connective tissue sheets termed myosepta. The linea alba is connective tissue separating left and right myomeres midventrally (Figures 3.18 and 3.19). The myomeres are complex internally, extending further anteriorly and posteriorly than their margins at the surface of the body.
To examine the appendicular muscles, skin portions from the dorsal and ventral surfaces of a pectoral fin and note the musculature revealed. The pectoral abductor lies dorsally (Figure 3.17) and the pectoral adductor lies ventrally (Figure 3.18). Abductor and adductor are terms that describe the actions of muscles. Abductor muscles pull a structure away from the midventral line; adductor muscles pull toward the midventral line. The pelvic adductor and abductor may be found associated with the pelvic fin. However, be careful if you decide to remove the skin from the ventral surface of the pelvic
FIGURE 3.16 Sketch of the shark in left lateral view showing guidelines for cutting the skin.
fin of a male.
The siphon (see page 62; Figure 3.33), a structure associated with the reproductive system, lies on the ventral surface of the fin musculature.Muscles of the Head and
Branchial Region
Examine next the musculature of the head and branchial region. Ideally, the head would be skinned from snout to the pectoral girdle, but this is not necessary. Instead, skin only half the head (as in Figure 3.16, flap 2). Make an incision through the skin middorsally between the eyes that extends posteriorly to the level of the pectoral fin. Continue the incision ventrally to the dorsal edge of the fifth pharyngeal slit. The skin may be removed from around the pharyngeal slits, but this is difficult and timeconsuming, as the skin adheres tightly here. This effort can be avoided without missing much anatomical detail by leaving a rectangular flap around the slits, as follows (Figure 3.16, flap 2). From the fifth slit, cut anteriorly to the first slit, ventrally along the anterior margin of this slit, and then posteriorly to the ventral end of the fifth slit. Continue the incision ventrally around the body to about 2 cm past the midventral line. Do not cut midventrally. Return to the beginning of the incision between the eyes and cut posteroventrally around the eye to the angle of the mouth. Proceed along the edge of the jaw until you reach ventrally approximately to the same level as the posterior cut. Separate the skin from the body using the methods described above for the axial musculature.
Branchiomeric Musculature
On exposing the musculature, view the shark in lateral view (Figures 3.17 and 3.37) and locate the spiracle. There are two muscles anterior to it, but careful dissection is required to separate them, so proceed cautiously. The small spiracularis is on the anterior wall of the spiracular valve. The larger levator palatoquadrati lies anterior to it. The muscles pass from the otic capsule to the upper jaw and serve to lift or stabilize it. The levator hyomandibulae lies posterior to the spiracle (Figures
FIGURE 3.17 Head, branchial region, and anterior part of the trunk of the shark in left lateral view, skinned to reveal musculature.
3.17 and 3.37). It extends between the otic capsule and hyomandibular and compresses the pharyngeal pouches. Ventral to these muscles and just posterior to the angle of the mouth is the large adductor mandibulae, which extends between upper and lower jaws and closes the mouth (Figures 3.17 and 3.18). Two other structures may be noted briefly at this time. The conspicuous hyomandibular nerve passes across the posterior part of the levator hyomandibulae toward the spiracle. The afferent spiracular artery passes deep to the nerve, and should be injected with red latex.
Posterior to the muscles just discussed are the five superficial constrictors, comprising a block of muscles that surrounds the pharyngeal slits (Figures 3.17-3.19). The constrictors are subdivided into dorsal and ventral portions. They are arranged sequentially from front to back and separated by vertical connective tissue partitions termed raphes, which look like white lines on the muscle surface. The first constrictor, more complex than the others, includes the dorsal and ventral hyoid constrictors, which lie between the adductor mandibulae and the first pharyngeal slit. Raphes extending dorsally and ventrally from the first slit separate the hyoid constrictors from the first of four superficial constrictors. The dorsal and ventral superficial constrictors, properly constrictors 3-6, are similar in form and extend between the raphes associated with each slit. The constrictors, which compress the branchial region, extend medially. Superficially, however, they overlap one another so that only their outermost portions are visible. The triangular cucullaris lies dorsal to the constrictors. Dorsal to the cucullaris are the dorsal longitudinal bundles of the epibranchial muscles, which attach anteriorly to the back of the chondrocranium and represent the anterior continuation of the epaxial musculature into the head. The cucullaris arises from fascia covering the longitudinal bundle and inserts on the epibranchial of each branchial arch and the scapular process of the pectoral girdle.
Examine the ventral surface of the shark (Figure 3.18).
The intermandibularis lies between the Meckel’s cartilages. Composed of left and right halves separated midventrally by a raphe, the fibers of the intermandibularis run posteromedially from mandibular Meckel’s cartilage toward the midline. The muscle is some 2 mm thick, but posteriorly it may be lifted from the underly-
FIGURE 3.18 Branchial region and anterior part of the trunk of the shark in ventral view, skinned to reveal musculature.
ing muscle. Carefully cut through the Intermandibularis parallel to and about 2-3 mm to one side of the raphe. A second, thin, and less extensive muscular sheet lies deep to it. This is the interhyoideus which, as its name implies, extends between the cartilages of the hyoid arch, specifically the ceratohyals. The muscle adheres tightly to the intermandibularis. It is difficult to separate them, but attempt to do so from the anterior end of the interhyoideus. The fibers of the interhyoideus are less obliquely oriented and do not extend as far anteriorly as the intermandibularis. The hyomandibular nerve may be seen lying ventrolaterally on the interhyoideus (Figure 3.18). If it becomes too difficult and time-consuming to separate the muscles, you may instead view the interhyoideus by cutting through it as well and reflecting the sheets (Figure 3.19).
The muscles discussed in this section are part of the branchiomeric musculature (there are others, but they lie more deeply and will be studied shortly). Now that their distribution has been noted, their relationships may be summarized. The muscles are each associated with particular visceral arches. Presumably in early vertebrates there were at least seven such arches, each
FIGURE 3.19 Branchial and hypobranchial musculature of the shark in ventrolateral view.
supporting a muscular septum that lay between pharyngeal slits.
The musculature was probably a repeating series of units, one for each arch, with each in the series resembling the relatively simple arrangement of a typical branchial arch in the shark. During the evolution of jawed vertebrates, the two most anterior arches were modified into the mandibular arch, which forms the upper and lower jaws, and behind it, the hyoid arch, which supports the jaws. The musculature associated with these arches was thus modified to meet the new functions of these arches. In the shark, the muscles of the mandibular arch are the levator palatoquadrati, spiracularis, adductor mandibulae, and intermandibu- laris. The levator hyomandibulae, hyoid constrictors, and interhyoideus belong to the hyoid arch. Each of the branchial arches, except for the last, has the typical constrictor setup that has just been described. The constrictors are differentiated into various parts; only the more superficial portions have been examined so far.The deeper portions require further dissection. Insert scissors into the third slit and cut vertically through the dorsal and ventral superficial constrictors so that the slit may be spread open. You will thus be able to view the interbranchial septa of the third and fourth branchial arches. Compare your specimen with Figures 3.20 and 3.23. Note that most of each septum is covered by gill lamellae. This portion of the septum is formed by the interbranchial muscle. Its fibers are circularly arranged. Carefully remove, by scraping, the lamellae from one septum to see them. The interbranchial is a deep portion of the constrictor musculature, and the superficial branchial constrictor lies lateral to it. Next snip frontally and completely through the middle of a septum and locate the cartilaginous arch. The small circular section of muscle that has been cut is the branchial adductor, a short muscle extending between the cerato- branchial and epibranchial that represents a deep derivative of the constrictor sheet.
Using a probe, separate the epibranchial musculature from the cucullaris and levator hyomandibulae and push these regions apart to expose the anterior cardinal sinus, which is part of the cardiovascular system (Figure 3.21). On the medial side of the sinus lie the dorsal surfaces of the branchial arches. Pick away connective tissue to expose the muscles, but do not injure the nerves that lie in this region. Find two sets of small, thin muscles. The strap-like and elongated dorsal interar- cuals lie between successive pharyngobranchial cartilages of branchial arches 1-4. Lateroventral to these lie the shorter and wider lateral interarcuals, which run between the epibranchial and pharyngobranchial cartilages of each of branchial arches 1-4.
Hypobranchial Musculature
Return to the ventral surface (Figures 3.18 and 3.19). If you have not already cut through the interhyoideus, do
FIGURE 3.20 Section through branchial septa of the shark.
so now and reflect it. The muscles visible between the hypaxial muscles and the lower jaw constitute nearly all of the hypobranchial musculature. They may be divided into a prehyoid group, including the Coracomandibular, and posthyoid group (see below). The coracomandibu- lar is the long, midventral, and nearly cylindrical muscle exposed on reflection of the intermandibularis and interhyoideus. It extends between the lower jaw and the muscles posterior to it, which attach to the coracoid bar. Cut the coracomandibular near its posterior end, reflect it, and note the dark, pinkish thyroid gland deep to the muscle’s anterior end (Figure 3.19).
The remaining muscles belong to the posthyoid group. Deep to the coracomandibular lie the elongated, paired Coracohyoids (Figures 3.18 and 3.19), which insert anteriorly on the basihyals. Posteriorly the coracohyoids are continuous with the broader, nearly triangular coracoarcuals, which lie medially between the ventral superficial constrictors and anterior to the hypaxial musculature. Cut the ventral superficial branchial con- trictor by snipping, on the same side that is skinned, from the last branchial slit to (but NOT through) the posterolateral end of the coracoarcual. Then dissect forward between the coracoarcual and ventral superficial constrictor to separate these muscular regions. Spread them apart to reveal the Coracobranchials (Figure 3.19), a series of five muscles that fan out from the coracoid bar, the coracoarcuals and the walls of the pericardial cavity to the ceratobranchial and basibranchial cartilages. Do not attempt to dissect these muscles at this point; doing so would require the removal and destruction of structures that have yet to be seen.
The names of the hypobranchial muscles contain the common component “coraco-” that refers to the coracoid bar, but only the coracoarcuals and parts of the coracobranchials attach directly to this structure. The others gain an indirect attachment by way of the coracoarcuals.
FIGURE 3.21 Blowup of branchiomeric and epibranchial musculature of the shark in right lateral view. The epi- branchial musculature has been dissected to reveal the interarcual muscles.
Key Terms: Muscular System
adductor mandibulae afferent spiracular artery
branchial adductor coracoarcuals coracobranchials coracohyoids coracomandibular cucullaris dorsal hyoid constrictors
dorsal interarcuals dorsal superficial constrictor
epaxial musculature epibranchial
horizontal skeletogenous septum hyomandibular nerve hypaxial musculature interbranchial interhyoideus intermandibularis lateral interarcuals levator hyomandibulae levator palatoquadrati linea alba myomeres myosepta pectoral abductor pectoral adductor
raphe siphon spiracularis
ventral hyoid
constrictors ventral superficial
constrictor