MUSCULATURE

Dissect the skin off the head, neck, and body to back of the pectoral fins. Observe first the musculature of the dorsal side of the neck and of the body back of the bases of the pectoral fins, noticing that it is composed of narrow, zigzag bands, called myomeres. Where these are fully developed they extend from the mid-dorsal to the mid-ventral line. Note carefully the relation of corresponding myomeres of the two sides, the exact course of a single myomere, and the direction of the muscle fibres in a typical myomere. Observe also that the muscles above the level of the vertebral column form a thick mass, which is frequently referred to as the dorsal musculature; the muscle below this level may be correspondingly referred to as the ventral musculature. As the muscles described below are dissected the mechanical effect of each should be determined.

Musculature of the head and neck. On the lateral and ventral surfaces of the neck the primary relations of the myomeres are much modified by the development of numerous special muscles, yet here and there traces of the metameric arrangement still show. Immediately beneath the skin is a thin sheet of muscle covering most of the ventral and lateral surfaces of the throat as far back as the pectoral girdle. On the ventral surface a triangular space is left in front of the pectoral bar; on the sides of the neck the sheet extends back to the last gill-cleft; dorsally, it reaches to the upper extremities of the gill pouches. This is the constrictor superficialis muscle. It is attached to fasciae dorsally and ventrally, and to the extra-branchial cartilages.

The constrictor superficialis consists of six metameric segments. The four posterior ones are distinctly limited by the gill-slits and extra-branchial cartilages. The second is anterior to the first gill slit, the largest of all, with distinct dorsal and ventral portions extending forward above and below the jaws. The first is recognized as consisting of two distinct parts, on the dorsal and ventral surfaces of the head. The dorsal portion is a small curved muscle on the anterior wall of the spiracle, extending from the external surface of the auditory capsule to the inner surface of the lower jaw. It lies close against the levator marillae superioris (see below). On the ventral surface of the throat the posterior constrictor muscles of the two sides are separated by a large triangular area. In front of this the ventral portions of the first and second constrictors meet in a median aponeurosis, from which their fibres extend transversely, those of the first to the mandibular cartilage, those of the second to the hyoidean cartilage. The first constrictor lies ventral or superficially to, and largely covers the second. Reflect the first constrictor from the aponeurosis outward, and demonstrate the two layers of muscle. It will be noted that the second to sixth constrictors consist of united dorsal, lateral and ventral portions, while the first is reduced to widely separated dorsal and ventral parts.

On each side of the head, just outside the angle of the mouth, is a large, thick muscle arising from the lateral surface of the cranium, and inserted upon the outer surface of the mandible, the adductor mandibularis.

In front of the small dorsal constrictor superficialis 1, and scarcely separated from it, is the strong levator maxillae superioris, which extends from the lateral surface of the auditory capsule to the dorsal edge of the palato-quadrate cartilage.

Eugaleus. The dorsal portion of the constrictor superficialis 2 reaches above the spiracle to the postorbital process. Reflecting it, the adductor hyomandibulae is seen behind the spiracle, arising from the upper part of the side of the auditory capsule and inserted on the end of the hyomandibular cartilage. The levator palpebrae inferioris arises under the origin of the levator hyomandibulae, and passes forward and downward between the spiracle and postorbital process, to insert in the posterior end of the lower eyelid. The depressor palpebrae superioris arises from the fascia dorsal to the spiracle, passes mediad to the levator palpebrae inferioris, upward and forward, to insert in the posterior end of the upper eyelid. Remove these muscles. The infra-orbital canal passes mediad to the muscles of the eyelids. The levator maxillae superioris lies between the spiracle and the orbit. Behind it is a small slip of muscle extending from the anterior wall of the spiracle to the lateral surface of the auditory capsule which seems to represent the constrictor superficialis dorsalis 1.

A thin sheet of muscle covers the anterior face of each inter-branchial septum. At the surface these pass into the constrictor superficialis, and are evidently portions of the latter muscle, though they are named the musculi interbranchiales.

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.

The circular muscles of the oesophagus are strongly attached to the last pharyngo-branchial.

Musculature of the pectoral fin. The dorsal muscle of the fin (levator-retractor) arises mostly from the scapular portion of the girdle, with a small part arising from the fascia covering the lateral musculature of the body. It is attached in fasciculi to the dorsal surfaces of the cartilaginous rays. The ventral muscle (depressor-protractor) arises from the median portion of the girdle and is inserted upon the cartilaginous rays in similar fasciculi. A portion of the lateral body muscles is inserted upon the scapular portion of the girdle.

Musculature of the pelvic fin. Ventral surface: An adductor muscle has origin upon the postero-lateral edge of the girdle; it is inserted upon the antero-medial surface of the basal cartilage of the fin. The depressor muscle consists of small fasciculi, each corresponding to a cartilaginous ray. They arise from the postero-lateral surface of the basal cartilage and are inserted upon the distal extremities of the rays.

Dorsal surface: The abductor arises from the fascia covering the trunk muscles, and inserts upon the fascia covering the intrinsic muscles of the dorsal side of the fin. These latter (levatores) are arranged in exactly the same manner as the fasciculi of the depressor.

Musculature of the dorsal fins. A sheet of muscle is attached to each side of the anterior dorsal fin, extending nearly up to the bases of the dermal fin-rays. This muscle passes downward between the dorsal body musculature of the two sides. Part of the fibres arise from the fasciae covering the medial surfaces of the body muscles, part from the basal cartilage of the fin itself. They are inserted upon the lateral surfaces of the broad cartilaginous fin-rays. The muscles of the posterior dorsal fin are exactly similar in arrangement.

Musculature of the caudal fin. There is no special musculature for the dorsal portion. A narrow, band-like muscle is found on each side of the ventral portion, widest above the triangular ventral lobe. The fibres of this muscle arise upon the flattened, expanded ends of the haemal spines. They pass obliquely backward and upward to be inserted in the fascia underlying the skin.