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

by Lawrence Edmonds Griffin

A large nerve which crosses the floor of the orbit, beneath the eyeball, consists of the maxillary trunk of the trigeminal and the buccal trunk of the facial nerve. These remain associated, even into the small branches. Near the anterior margin of the orbit the maxillary-buccal trunk divides into three parts; the smallest and outer one passes to the surface lateral and anterior to the eye. The other two dip downward and pass in front of the jaw to the ventral surface of the snout. Reflect the skin of the ventral surface of the snout, and by dissection expose these nerves as they emerge from the orbit. The larger branch runs forward close to the median line of the snout, giving off numerous twigs; the other, which appears to be a pure trigeminal branch, is distributed near the angle of the mouth. The fibres of the maxillary trunk supply the skin, while those of the buccal go to the canal organs and ampullae of Lorenzini.

The mandibular trunk of the trigeminal nerve arises beneath and behind the maxillary. It passes outward in front of the levator maxillae superioris muscle, sending a few twigs into this muscle, and turns downward over the palato-quadrate cartilage. It divides here, one part entering the adductor mandibularis muscle, the other passing downward along the edge of the mandible, innervating the skin of the lower jaw and the first ventral superficial constrictor muscle.

The mandibular and maxillary-buccal trunks of Eugaleus are united until they approach the edge of the orbit, and the palatine branch is much larger; otherwise the trigemino-facial branches are much as in Squalus.

The hyomandibular trunk of the facial nerve can be found just beneath the skin behind and close to the spiracle. From here it can be followed back to the brain. It arises from the ventral part of the trigemino-facial root, emerging from the cranium through the hyomandibular canal. It divides into a number of branches just beyond the spiracle:

1. The external mandibular branch consists of two portions, a small anterior nerve extending antero-ventrally to the canals above and below the angle of the mouth, and a larger nerve which passes laterally and suddenly breaks up into a brush of twigs which innervate the hyoidean group of ampullae.

2. The internal mandibular branch arises at about the same level as the external mandibular, but under it, passes inward around the edge of the hyoid cartilage, under the adductor mandibularis muscle, and then forward along the mandibular cartilage.

3. The hyoid branch separates from the hyomandibular trunk at about the same level as the preceding nerves, and then passes, deep in the tissues, around the angle of the jaw to the ventral side where it is distributed to the superficial constrictor muscles. Several nerves pass from the hyomandibular trunk and the hyoid branch to the dorsal superficial constrictors.

4. The palatine branch springs from the base of the hyomandibular trunk inside the hyomandibular canal. It passes outward and forward, dividing into numerous branches which innervate the mucous membranes of the mouth. It can be traced completely later.

One or more small nerves proceeding to the pseudobranch and anterior wall of the spiracle arise near the point of origin of the palatine branch.

The otic nerve, passing from the root of the facial nerve to the postorbital canal, is not likely to be found in this dissection.