The muscular bellies of the pronator and the flexor muscles arising from the ulna and internal condyle of the humerus have Been cut through and reflected to show this insertion of the biceps.
1 Bischoff gave this muscle the convenient name of latissimo-condyloideus, and was followed by Dr. H. C. Chapman, in his Memoir on the Structure of the Gorilla, Proc. Acad. Sci. Philadelphia, I878.
This dissection contrasts with a similar dissection of the human subject in the imperfection of the clavicles; in the absence or rudimentary condition of the omo-hyoid; in the presence in the neck of two additional muscles, the acromio-basilar, n, and the cleido-occipital, m; in the formation of a compound 'cephalo-humeral' muscle, as in many other mammalia, by the physiological combination of the cleido-mastoid, k, the cleido-occipital, m, and the acromio-basilar, n, with the deltoid, f; in the prolongation of the sterno-clavicidar; g, and stemo-scapular muscles, h and i, the homo-logues of the subclavius, on to the spine of the scapula at j, whence they act in the way of slinging up the horizontally-carried trunk; and, finally, in the greater size of the cervical platysma, x, and the development of a cutaneous muscle, o, the 'dermohumerien', s, 'costoalaris,' s, 'brachiolateral, in the regions of the back and flanks. The cleido-occipital, k, the acromio-basilar, n, and the 'latissimo-condyloideus', e, are more or less frequently represented by muscular varieties occurring in the human subject, and the absence of any scalenus anticus in the Rabbit is paralleled in man by the occasional perforation of the fibres of that muscle by the upper factors of the brachial plexus, one of which, the fifth cervical, has been observed to pass entirely in front of the muscle.
The digastric, u, the biceps, l, and the pectoralis minor, e, present points of difference, stated above, in which the Leporidae and very many other mammals coincide with each other and differ from man. The rectus abdominis is very usually prolonged in mammalia lower than man up to the second or first rib, and in the rabbit up even to the base of the manubrium; when thus prolonged, it is known as the 'rectus thoracis' It is frequently crossed at its upper end, as here, by a muscle known as the 'sterno-costalis,' passing downwards and covering the front of the rectus just as the external oblique does that of the rectus abdominis. It may be better however to speak of the sterno-costalis as being a lateral efflorescence of the rectus1.
On the other hand, the arrangement and relations of the various structures of the lowly organised mammal here figured are sufficiently similar to those of man to cast considerable light upon some even of the more intricate points of anthropotomy. Among them we may specify the occurrence of certain varieties in muscles, the relations of the deep cervical fascia, B and the insertions of the pectoral muscle, c and d.
1 For a discussion of the homologies of the rectus thoracis and the sterno-costalis, see Professor Turner, Journal of Anat. and Physiol. May, I867, pp. 247-253; May, I868, pp. 392-394; Wood, Ph. Tr. for I870, pp. 110-112, vol. I60. These two muscles are not lettered in this figure, but are seen to form a triangle with part of the first rib for its base and with its apex covered by the sterno-clavicular muscle g. For a figure showing the rectus abdominis giving off a part of the pectoral, see Ecker, Anatomie des Frosches, 1864, p. 95.
a. Bifid upper lip concealing muffle. See p. 26, y', infra.
Fig. i. - Dissection of Superficial Muscles in Anterior Regions of Head, Neck, and Thorax in Rabbit (Lepus cuniculus); nearly Natural Size.
b. Sterno-mastoid muscle, arising from the prolongation of the manubrium into the neck and inserted into the mastoid.
c. Greater pectoral muscle of left side, arising along nearly the entire length of the sternum from the manubrium anteriorly down nearly to the leaf-shaped cartilage ending it posteriorly.
c. Part of the origin of this muscle on the right side reflected.
c". Pouch-like insertion of it into the humerus, the fibres from the lower part of the sternum forming the posterior, those from the upper the anterior portion of the pouch; the former passing to the inner; the latter to the outer tuberosity of the humerus, and the outwardly-looking pectoro-deltoid ridge.
d. Smaller pectoral muscle arising from the outer side of the keel-shaped manubrium superficially to the upper fibres of the greater pectoral, inserted together with fibres of that muscle and of the clavicular portion of the deltoid into the fascia covering the biceps, and reaching by tendon nearly to the lower end of the humerus at d". d'. Tendon of origin of smaller pectoral of right side cut short. d". Tendon of insertion of smaller pectoral of same side. e- Third pectoral muscle, corresponding to the pectoralis minor of anthropotomy, as being similarly innervated, but inserted not into the coracoid, but into the head of the humerus, and only by a few fibres into the costo-coracoid membrane.
f. Deltoid, a bicipital muscle, its anterior head, supplied by the circumflex nerve, arising from the outer end of the clavicle and the fibrous tissues external to that insertion covering the shoulder-joint, and the posterior head arising from the apex of the acromion and the anterior edge of the metacromial process of the scapula. The external tuberosity of the humerus appears in the interval between the two muscular bellies.
g. Origin of sterno-clavicular muscle, from the sternum down to the level of the sixth rib, from the cartilage of which it receives some fibres, at a deeper level than that of the origins of the three pectorals, but superficially to the plane of the rectus thoracis and of the sterno-costal muscle, which are seen between the origin of the sterno-clavicular muscle and the plexus of brachial nerves to form, with the first rib, a triangle with its apex pointing downwards and inwards.