A Guide for the Dissection of the Dogfish (Squalus Acanthias)

by Lawrence Edmonds Griffin

In front of the medulla, and overlapping its anterior extremity, is a large oval organ, the cerebellum (metencephalon). Ventral to the cerebellum, each side of the medulla is expanded in an ear-shaped lobe, the corpus restiformis. Anteriorly, the cerebellum overlaps a pair of rounded lobes, the optic lobes, which together form the dorsal portion of the midbrain (mesencephalon).

In front of the optic lobes are two slightly larger lobes united in their posterior portions but separated anteriorly, the cerebral lobes or hemispheres. Together they constitute the prosencephalon. (The prosencephalon is not divided in Eugaleus.) Between the mesencephalon and the prosencephalon is a depressed region belonging to the brain-stem, the diencephalon (thalamencephalon), from which the epiphysis arises. The roof of the diencephalon also is very thin and is frequently broken during the exposure of the brain. The cavity seen within the diencephalon is the third ventricle.

Stalked bodies arising from the antero-lateral angles of the cerebral hemispheres are the olfactory lobes. The portion of the brain including the cerebral hemispheres and the olfactory lobes constitutes the telencephalon.

Dissection of the internal ear. The structures composing this organ lie in the projecting cartilage at the side of the medulla (auditory capsule). Remove the cartilage of the auditory capsule in thin slices and bit by bit, following the ductus endolymphaticus to the membraneous labyrinth. Dissect away the surrounding cartilage leaving the membraneous canals in place, until the entire labyrinth is exposed. The membraneous labyrinth consists of a large central sac (utriculo-saccular chamber) into which the endolymphatic duct opens, and three membraneous tubes (semicircular canals) external to the chamber but communicating with it in various ways. Two, one anterior and the other posterior to the sacculus, lie in a nearly vertical plane (anterior and posterior semicircular canals); one is external to the sacculus and lies in a nearly horizontal plane (horizontal or external semicircular canal). At the ventral ends of the vertical canals are nearly spherical enlargements called ampullae. The ampulla of the horizontal canal is at its anterior end. The dorsal ends of the vertical canals open near each other into the upper part of the utriculo-sacculus. The ventral extremity of the anterior vertical canal and the anterior extremity of the horizontal canal open beside each other into an anterior projection of the sacculus. The ventral extremity of the posterior vertical canal opens into the posterior and lower part of the sacculus. The posterior extremity of the horizontal canal opens into the posterior side of the sacculus. During life the utriculo-sacculus and the semicircular canals are filled with a lymphatic fluid, and the sacculus contains a large calcareous ear-stone (otolith), which is usually dissolved by the formalin used in preserving the dogfish.

Whitish patches of thickened sensory epithelium may be seen in the ampullae (cristate acusticae) and in the utriculo-saccular chamber (maculae acusticae). Branches of the eighth nerve can be followed to all these areas.

A projection of the ventral wall of the utriculo-sacculus is the lagena, the rudiment from which the cochlea of higher animals developed. It also contains a macula acustica.

External features of the eye. Observe the transparent cornea covering the external surface of the eye; the dark ring of the iris; the central opening in the iris, the pupil; the conjunctival sac surrounding the external half of the eyeball. Cut away sufficient of the upper wall of the cartilaginous orbit to expose the eyeball and its muscles. Note the considerable amount of soft connective tissue around the eye and explore the orbital sinus (p. 13). Take notice of the following nerves, in order to ensure their preservation until the time comes to trace them more completely. A large nerve crossing the medial side of the orbit, the superficial ophthalmic; a nerve leaving the cranium opposite the optic lobe, passing under the superficial ophthalmic to the anterior muscle of the eyeball, the trochlear; several long ciliary nerves passing to the eyeball; several other nerves visible in the deep angle of the orbit.

Six muscles move the eye. Four of these arise close together at the deep postero-medial angle of the orbit. Diverging, they are inserted upon four sides of the eyeball, and from the position of their insertions are named the superior, posterior, inferior, and anterior recti. Two muscles arise from the antero-medial angle of the orbit, the superior and inferior oblique muscles.

Between the recti muscles can be seen a mushroom-shaped stalk of cartilage, the ophthalmic peduncle; the eyeball rests against its expanded end. (There is no peduncle in Eugaleus.)

The cranial nerves. The cranial nerves are twelve pairs of nerves arising from the brain, and thus distinguished from the spinal nerves which arise from the sides of the spinal cord. They are distributed chiefly to the head and neck, though branches of the vagus nerve go to the viscera and to the sense organs of the lateral line. Since the nerves are all paired, the distribution of both nerves of a pair being alike, the descriptions will mention but one nerve of a pair. As the cranial nerves are traced dissect away the sides of the cranium down to the foramina penetrated by the nerves, and follow each nerve from its origin on the brain to the parts innervated by it. Features of the dissection which are not found in tracing the nerve of one side should he sought on the other side.