Section VII—Sensory Organs

Ampullae of Lorenzini

The ampullae of Lorenzini (Figures 3.15 and 3.35) are a modified part of the lateral line system (see below) and primarily sensitive to electrical fields (they can help a shark sense prey by detecting the electrical fields gener­ated by activities of the prey).

Lateral Line System

The lateral line system is composed of a series of canals and sensory receptors that provide sensory information by detecting disturbances in water. The position of the lateral line canal, extending midlaterally along the length of the body and into the tail, was noted earlier (page 37). The canal lies within the skin. Make a cut in the skin, perpendicular to the canal, and examine the cut surface. The canal appears as a small hole. The canal leads to various other canals in the head. If you skinned the head to find the ampullae of Lorenzini, you should be able to observe, about midway between the eye and sagittal plane, a groove for one of these canals (the supraorbital) heading toward the snout. Another canal (the infraorbital) passes down behind the eye and then forward toward the snout. Trace the canals on the head, which are named in Figure 3.35, if time permits.

Nose

The olfactory sacs function in olfaction (Figures 3.36, 3.41-3.44). Reexamine the nares and note that each is subdivided into lateral incurrent and medial excurrent apertures that direct the flow of water into and out of the olfactory sac. Skin the region adjacent to one of the nares.

Eye

Lift the edge of an eyelid and note its soft and thin inner surface, which consists of a portion of soft, modified skin. The skin folds over and fuses onto the surface of the eyeball as the transparent conjunctiva. Begin the exposure of the eye by removing the soft tissue of the upper eyelid. Then cut two or three very thin slices from the top of the supraorbital crest, holding the scalpel hor­izontal. This will clearly reveal the semicircular margin of the orbit, the space housing the eyeball and various other structures. Continue to remove the crest. Make a few vertical slices and follow the semicircular margin as you make your way toward (but not to) the medial wall of the orbit. Stop after you have removed a thickness of about 3 mm of cartilage. The eyeball is covered by a gelatinous connective tissue. Carefully remove the latter.

Observe the eyeball’s medial surface as well as some of the extrinsic muscles of the eyeball that extend between it and the orbital wall. Identify the superficial oph­thalmic nerve, a strand about one-third the width of the

FIGURE 3.35 Head of the shark partially skinned in (a) dorsal and (b) ventral views to reveal the lateral line system.

muscles, which passes anteroposteriorly along the dor- somedial wall of the orbit. Once the nerve is located, continue to remove cartilage to expose the orbit more completely, but do not cut into the otic capsule. Remove any other connective tissue, but avoid injuring the nerves in the orbit. You now have a dorsal view of the orbit and its contents (Figure 3.36).

The two groups of extrinsic eye muscles are the obliques and rectus muscles.

The four rectus muscles radiate toward the eyeball from the posteromedial wall of the orbit. Three are clearly evident in dorsal view. The lateral rectus muscle extends almost directly laterally and attaches to the posterior surface of the eyeball. The medial rectus muscle passes anteriorly and attaches to the medial surface of the eyeball. The dorsal rectus muscle extends between the lateral and medial rectus muscles and attaches to the top of the eyeball. The fourth muscle, the ventral rectus muscle, will be seen more completely once the eyeball has been removed. For now, probe gently between the dorsal and lateral rectus muscles to find the ventral rectus. Avoid injuring the thin nerves that wind around the muscles.

By probing between the medial and dorsal rectus muscles, observe how the optic pedicle provides support for the eyeball (Figure 3.37). Tug the eyeball laterally and identify the deep ophthalmic nerve, a thin, whitish strand passing anteroposteriorly through the orbit and adhering to the medial surface of the eyeball. Using needle and forceps, separate the nerve from the eyeball. Next, look deep within the orbit, where the ventral oblique and medial rectus muscles converge toward the eyeball. Note the thick optic nerve extending from the medial wall of the orbit laterally to the eyeball.

FIGURE 3.36 Head of the shark in dorsal view showing the chondrocranium shaved down to reveal the brain, sensory organs, and nerves.

Remove the eyeball by cutting the oblique and rectus muscles near their insertions. Be careful not to injure the thin, whitish strands that pass to or near the muscles.

Observe the orbit in lateral view and review the struc­tures already identified: oblique and rectus muscles, superficial and deep ophthalmic nerves, optic nerve and optic pedicle (Figure 3.37). Lift the ventral rectus and observe the thick infraorbital nerve passing anteriorly and slightly ventrally (Figures 3.36 and 3.37). It is nearly as wide as any of the extrinsic muscles. Carefully picking away tissue where the nerve meets the anteroventral orbital wall reveals that the infraorbital nerve subdivides. As the nerve passes through the orbit, it crosses over the preorbitalis muscle (Figure 3.41). The preorbitalis muscle is about twice as wide as the extrin­sic muscles and passes anteromedially across the floor of the orbit.

Trace the path of the superficial ophthalmic nerve and note the orbital process of the palatoquadrate (Figure 3.37; see also page 28 and Figure 3.3) that lies medial to the posterior half of the nerve. Next, follow the course of the deep ophthalmic nerve. It passes forward in the orbit, ventral to the dorsal rectus and dorsal oblique muscles. Lift the dorsal oblique muscle and observe the deep ophthalmic nerve and stalk of the optic pedicle. Just ventral to the deep ophthalmic is the ocu­lomotor nerve. The latter passes almost immediately ventrally and around the margin of the ventral rectus. The lateral rectus muscle is innervated by the abducens

FIGURE 3.37 Head of the shark in left lateral view. The head has been skinned and the left eyeball removed to show muscles and nerves in the orbit.

nerve, which you will probably not see now (see page 74). Gently tug the dorsal oblique muscle anteriorly and note the thin strand-like trochlear nerve passing to it.

Examine the eyeball removed earlier (Figure 3.38) and note the insertions of the extrinsic muscles. The outer surface of the eye is the fibrous tunic. Its lateral portion is modified into the transparent cornea. The remainder is the mainly cartilaginous sclera, which helps support the eyeball. The pigmented structure visible through the cornea is the iris, which has a circular opening, the pupil, at its center, through which light enters the eye. Section the outer eyeball transversely and refer to Figure 3.39 to help identify the following structures. The lens is the hard spherical structure. Carefully cut through the pigmented tissue, the ciliary body (see below), that attaches to the lens, but only cut around half of the cir­cumference of the lens so that it remains attached to the half of the eyeball that you will examine. The vascular tunic is the dark pigmented layer internal to the sclera, and most of it consists of the choroid. The medial wall of the eyeball, where the optic pedicle attaches, con­tains the suprachoroidea, a vascular tissue between the choroid and sclera.

The iris, noted above, is a modified part of the vascular tunic. It extends between the cornea and the lens; the pupil is the opening in the iris. The iris contains some of the eye’s intrinsic musculature. These smooth muscles act to control the size of the pupil, thus regulating the amount of light that enters the eye. Another modified part of the vascular tunic forms the ciliary body, which holds the lens in place. It is the tissue you cut through to free the lens from one half of the eyeball. The ciliary body also contains some of the eyeball’s intrinsic mus­culature that here helps control the shape of the lens. The light-colored tissue internal to the choroid is the retina, the light-sensitive layer that contains the pho­toreceptors responsible for absorbing light.

FIGURE 3.38 Eyeball of the shark in lateral and medial views.

is an incomplete layer, extending to about the base of the ciliary body. It is not tightly interconnected with the choroid, and when the eye is cut open the retina tends to become detached. The large cavity of the eye medial to the lens is the vitreous chamber. It is filled by a gelati­nous mass, the vitreous humor, which helps maintain the eyeball’s shape and holds the retina in place. The smaller cavity lateral to the lens is subdivided by the iris into the anterior chamber and the posterior chamber, and filled with the watery aqueous humor.

Ear

The paired inner ears of the dogfish are organs of balance or equilibrium and are embedded in the otic capsules of the chondrocranium. Each ear consists of a series of ducts and sacs, collectively termed the mem­branous labyrinth. The ducts and sacs are suspended in a series of canals and chambers, the cartilaginous labyrinth, within the otic capsule. The membranous labyrinth is filled with a fluid termed endolymph. Move­ment of the endolymph within the canals leads to per­ception of the orientation and position of the body.

The membranous labyrinth is formed of three thin, semicircular ducts and the sacculus (Figures 3.40 and 3.41). The anterior and posterior semicircular ducts are vertically oriented. The third duct, the lateral semicir­cular duct, lies mainly horizontally. The sacculus, a large, triangular, sac-like structure, extends between

FIGURE 3.39 3-D cutaway illustration of the left eyeball of the shark in posterolateral view revealing internal structures.

the vertical ducts and medial to the lateral duct. The endolymphatic duct extends dorsally from the sacculus and reaches the exterior surface of the head through the endolymphatic pores. Two cylindrical chambers, the anterior and posterior utriculi, are closely associated with the sacculus. Each utriculus communicates with the sacculus by way of small openings. The ends of each duct attach to one of the utriculi. Those of the anterior and lateral semicircular ducts attach to the anterior utriculus, those of the posterior semicircular duct to the posterior utriculus.

The ear may be examined by shaving away the cartilage of the otic capsule. Before beginning, study a prepara­tion of the ears. Such preparations usually have the chondrocranium embedded in an acrylic block. The form and position of the canals and chambers, injected with red latex, are clearly discernable.

Start your dissection by removing skin and musculature from around an otic capsule. Using a fresh scalpel blade, shave away thin slices of cartilage from one of the otic capsules, beginning dorsally. The slices should be thin enough so that you clearly see the blade through the car­tilage. As you progress ventrally, you will probably need to remove more musculature laterally, as well as the spiracle. The dorsal portions of the anterior and poste­rior semicircular canals will become apparent through the cartilage. Carefully continue to remove cartilage to cut into the canals. The thin, tube-like semicircular ducts of the membranous labyrinth will then be exposed. As you proceed ventrally, you will also

FIGURE 3.40 Membranous labyrinth of the inner ear of the shark in lateral and medial views.

uncover the large central cavity housing the sacculus. The cavity lies medial to the lateral semicircular canal and increases in size ventrally. Careful dissection will reveal the sacculus, but it is often collapsed in the floor of the cavity. The position of the lagena, a posteroven- tral extension of the sacculus, may be discerned by probing the floor of the sacculus.

Key Terms: Sensory Organs

abducens nerve ampullae of Lorenzini anterior chamber anterior semicircular

duct

anterior utriculus

aqueous humor

choroid

ciliary body conjunctiva cornea

deep ophthalmic nerve dorsal oblique muscle dorsal rectus muscle endolymphatic duct eyeball

fibrous tunic

infraorbital nerve

iris

lagena

lateral line canal

lateral rectus muscle

lateral semicircular duct lens

mandibular nerve medial rectus muscle oculomotor nerve

olfactory sacs

optic nerve

optic pedicle

orbit

posterior chamber

posterior semicircular duct

posterior utriculus preorbitalis muscle pupil

retina

sacculus

sclera

FIGURE 3.41 Head and branchial region of the shark in dorsal view. Cartilage and soft tissue have been shaved down to reveal brain, nerves, and sensory organs. The right side auditory region has been shaved down farther ventrally than the left side. Left eyeball has been removed.

superficial ophthalmic

nerve suprachoroidea trochlear nerve vascular tunic

ventral oblique muscle ventral rectus muscle vitreous chamber vitreous humor