24. The quadratus labii inferioris ([p. 107]).
III. Muscles of the Fore Limb.
A. Muscles connecting the Arm with the Body.
—If the skin-muscles have been dissected, as above, cut the cutaneus maximus near its insertion, and reflect it toward the median dorsal line; in this way remove it completely. Remove the platysma in the same way. The first layer of body-muscles is thus exposed.
If the skin-muscles are not to be dissected, these may be removed with the skin. If the skin has not yet been removed, proceed as follows:
Beginning at the cranial end of the manubrium make two incisions in the skin, one passing to the lambdoidal ridge and the other to the spinous process of the thirteenth thoracic vertebra. Raise the triangular flap thus formed, taking up the skin-muscles with it.
1. The trapezius group of muscles ([pp. 115]-[117], and [Fig. 68], d, h, and j) is now exposed and may be dissected. Begin with the spinotrapezius ([Fig. 68], j). Read the description of the muscle, then raise its caudal border and work under it until its cranial border is reached and the middle of its inner surface is free. Then transect it, and reflect the two halves of the muscle toward the origin and insertion, clearing the fat, etc., from the inner surface of the muscle and the parts covered by it. Determine origin and insertion.
2. The acromiotrapezius ([Fig. 68], h). Dissect in the same manner, being careful not to injure the broad thin tendon which connects the two muscles across the middle line.
3. The clavotrapezius ([Fig. 68], d). Separate it carefully, from the cleidomastoid ([p. 120]). Transect and reflect, as before.
4. The occipitoscapularis ([Fig. 73], a, [p. 149]). Note the strong fascia which separates it from the deeper muscles of the neck. Be careful not to injure adjacent muscles in tracing this toward origin and insertion. Transect.