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

by Lawrence Edmonds Griffin

Above the constrictor superficialis, lying on the side of the neck between it and the dorsal musculature, is a broad trapezius muscle. It arises from the fascia covering the lateral surface of the dorsal musculature. Its fibres pass obliquely downward and backward, mediad to the posterior gill pouches, to insert upon the anterior edge of the scapular portion of the pectoral girdle. The anterior portion of the trapezius is also inserted upon the epibranchial of the fifth gill arch.

Just in front of the mouth is a pair of strong muscles (levator labialis superioris), each arising from the ventral surface of the cranium close to the median line. They pass into strong tendons which are inserted among the fibres of the ventral portion of the adductor mandibulae. The muscle mass in front of the mouth and the lower part of the adductor mandibularis thus form the two bellies of a digastric muscle, with the tendon between them.

[^[2]]Remove the ventral portions of the first and second superficial constrictors and clear the mass of muscles lying between the coracoid portion of the pectoral girdle and the mandible. Immediately in front of the girdle are two large muscles, the coraco-arcuales communes, whose fibres run inward and forward. These muscles cover the ventral surface of the pericardium, to the wall of which their median fibres are attached, while the lateral fibres are attached around the ventral ends of the gill arches.

[2]. The coraco-mandibularis, coraco-hyodeus, coraco-arcualis communis, and coraco-branchialis muscles should be dissected as a preliminary operation to following the ventral aorta and its branches.

In front of the coraco-arcuales communes are three large longitudinal muscles. The median, unpaired one, arising from the fascia between the coraco-arcuales communes and inserted upon the posterior surface of the lower jaw, is the coraco-mandibularis. The other two, which lie dorsal to and outside of the coraco-mandibularis, are the coraco-hyoidei. They arise from the fasciae covering the anterior ends of the coraco-arcuales communes and posterior parts of the coraco-branchiales, and insert upon the basihyal.

Dissect out the coraco-mandibularis and coraco-hyoidei, noting particularly the form and place of origin of the latter. Dorsal to the coraco-hyoidei are the first divisions of the right and left coraco-branchialis muscles, which arise from fascia covering the anterior ends of the coraco-arcuales communes, and are inserted upon the ventral extremity of the ceratohyal cartilage. Notice that they pass dorsad to the anterior branches of the aorta, and that the aorta itself can be exposed between them.

Remove the coraco-arcualis communis by dissecting it from the pectoral girdle and reflecting it forward. The other four divisions of the coraco-branchialis are now revealed, attached to the lateral surface of the pericardium and the lateral portion of the coracoid. The divisions of the muscle are clearly separated only near their insertions. The second, third, and fourth coraco-branchials are attached to the hypobranchial cartilages of the second, third and fourth visceral arches. The fifth division is inserted upon the lateral portion of the basibranchial and the expanded medial end of the fifth ceratobranchial.

The first aortic branch passes ventral to the first coraco-branchial. The second aortic branch passes between the first and second coraco-branchials. The third aortic branch passes between the second and third coraco-branchials. The fourth aortic branch passes between the third and fourth coraco-branchials.

Expose the dorsal ends of the gill arches by clearing away muscles and other tissues between the gill pouches and the spinal column. Two sets of four small muscles (interarcuales) will be found connected with the branchial cartilages. The second, third and fourth medial interarcuales extend from the posterior surfaces of pharyngo-branchial cartilages 1, 2, and 3, to the dorsal surfaces of pharyngo-branchials 2, 3, 4, and 5. The first medial interarcuale arises from the under surface of the cranium and inserts on the upper end of the first pharyngo-branchial.

The lateral interarcuales lie immediately below the medials. The first has a double origin, most of the fibres arising from the lower part of the posterior edge of the first pharyngo-branchial; a smaller bundle from the anterior edge of the second pharyngo-branchial. Its insertion is along the dorsal surface of the first epibranchial cartilage. The second and third lateral interarcuales are like the first, but since the fourth and fifth pharyngo-branchials are fused, the origin of the fourth lateral interarcuale is not divided.