Part 91

In transecting a muscle, work under it completely from one edge to the other (except in case of very wide muscles), then introduce the scissors or scalpel, and cut it by a smooth clean cut. Always leave the entire origin with one-half of the muscle, the entire insertion with the other half.

The student must be prepared to find in some cases considerable variations from the conditions described. The descriptions attempt to give an account of the usual structures, but specimens showing no variations whatever are undoubtedly rare.

DIRECTIONS FOR DISSECTING THE MUSCLES OF THE CAT.

I. THE SKIN-MUSCLES (pp. 93-96, and Fig. 62).

These will perhaps not usually be dissected. If they are to be dissected, proceed as follows: Make a ventral median longitudinal incision of the skin from the cranial end of the manubrium to a point opposite the crest of the ilium. Connect the cranial end of this incision with the middle of the lambdoidal ridge by a similar incision through the skin. Make another incision from the manubrium to the spinous process of the thirteenth thoracic vertebra. Connect the caudal end of the first incision by an oblique cut with the root of the tail. Make another incision from the convexity of the knee-joint along the fold of skin which connects the hind limb and body, as far as the incision which leads to the root of the tail. Make an incision surrounding the base of the forelimb. Now reflect these flaps one at a time, working in general from the ventral side toward the dorsal middle line. Take great pains not to take up the thin skin-muscle with the skin. In this way all of one side of the body will be uncovered. The cutaneus maximus (Fig. 62, b) and a part of the platysma (Fig. 62, a) will be thus exposed. Read and verify the descriptions of these muscles (pp. 93-96). The cranial portion of the platysma cannot be seen at this stage, but should be dissected in connection with the muscles of the face.

II. SUPERFICIAL MUSCLES OF THE FACE AND HEAD (pp. 96-107, and Figs. 63 and 64).

These will perhaps not usually be dissected by the student. They should be studied on a well-dissected laboratory preparation, if possible. If they are to be dissected, this should be done if possible on a fresh specimen, as it is very difficult to dissect them on preserved material. Proceed as follows:

The skin should be removed with care from the sides and top of the face and head, without injury to the external ear.

1. The platysma (p. 95, and Fig. 62, a). Determine its cranial attachments.

2. The intermedius scutulorum (p. 96, and Fig. 63, a).

3. The corrugator supercilii medialis (Fig. 63, b).

4. The orbicularis oculi (Fig. 63, c; Fig. 64, s).

5. The corrugator supercilii lateralis (Fig. 64, k).

6. Notice the frontoauricularis if it exists.

7. The levator auris longus (Fig. 63, g, g′). Transect and reflect.

8. The auricularis superior (Fig. 63, k). Transect.

9. The abductor auris longus (Fig. 63, m). Transect.

10. The abductor auris brevis (Fig. 63, l). Transect.

11. The epicranius (Fig. 63, h, h′). In dissecting this muscle the intermedius scutulorum and corrugator supercilii medialis may be cut by an incision parallel to the middle line but a short distance to one side of it.

12. The zygomaticus (Fig. 64, d). This and the two following muscles may usually be best seen running beneath the fibres of the platysma, without reflecting the latter. If necessary, however, the platysma may be transected and reflected, though this can be done only with great difficulty.

13. The submentalis (Fig. 64, c).

14. The depressor conchæ (Fig. 64, b).

The zygomaticus, submentalis, and depressor conchæ should now be transected one or two centimeters from the external ear. The corrugatores supercilii lateralis and medialis may be cut across between the eye and the ear, and reflected. There is thus exposed:

15. The frontoscutularis. Transect near the scutiform cartilage.

16. The adductor auris inferior (Fig. 64, o). Transect near its insertion.

The external ear is now attached to the head only by the external auditory meatus and the tragicus lateralis muscle. Read the description of the latter (p. 418, and Fig. 67, a), find it beneath the fat which surrounds the proximal portion of the external ear, and cut it as far ventrad as possible. Now cut across the cartilaginous auditory meatus as near to the tympanic bone as possible, and remove the external ear, preserving it for future study. (The remainder of the muscles of the external ear may be studied in connection with the study of the auditory organ.)

17. The orbicularis oris (p. 105 and Fig. 64, i).

18. The zygomaticus minor, if it exists (p. 105, and Fig. 64, e).