OCR Text |
Show 1866.] ON THE ANATOMY OF THE CRESTED AGOUTI. 397 Muscles of the Fore Limb. Pectoralis major. Perhaps the most striking difference between the posterior fibres of this muscle and that of the conjoined panni-culus carnosus is the darker colour of the former. The pectoralis major is extensive, but on the whole of moderate thickness. Although peculiar, we did not find it to answer the description given by Prof. Owen of the same in Capromys fournieri*; for it seemed to have four distinct origins, and as many insertions. The first and largest portion consists of a very broad sheet of muscular fibre arising from the posterior two-thirds of the sternum, and, as already stated, is intimately blended with the panniculus carnosus (see fig. 2, P. m. 1); while it is also much adherent to the posterior portion of the sterno -scapular muscle. Its insertion extends from halfway down the deltoid ridge, upwards as far as the tendon of the coraco-brachialis; and a small slip of its fibres passes over this tendon to the surface of the fascia covering the subscapularis. The second portion arises from the anterior fifth of the sternum, almost as far as the end of the manubrium. Its posterior border is closely connected with the anterior border of the first portion, so that these two together almost, if not quite, cover the sternal origin of the sterno-scapular. It is inserted into the shaft of the humerus, its whole length external to the insertion of the preceding portion (figs. 2 & 3, P. m. 2). The third portion is a narrow band of muscle arising from the manubrium, and inserted into the lower end of the shaft of the humerus, immediately external to the second portion of this muscle (figs. 2 & 3, P. m. 3). The fourth portion is a still narrower band, which arises from almost the outer end of the clavicle, and joining the third portion is » inserted in common with it; and it is this junction and common insertion with an undoubted part of the pectoralis major which decides us to regard it as part of the pectoral, and not as a part of the deltoid (figs. 2 & 3, P. m. 4). In the Hare, the Rabbit, and the Guinea-pig this muscle exists in a slightly modified condition from the above; and although the several portions spoken of are in them more adherent, nevertheless they are without much difficulty separable into nearly similar slips. What answers to the first and second portions in D. cristata may in them be regarded as but one sheet folded on itself at its sternal and humeral attachments. The insertion into the humerus of the third portion is neither so extensive nor so muscular in the three rodents in question. In Dasyprocta cristata the pectoralis minor is wanting, or included in the p. major ; the slip of the first portion of the latter muscle going to the subscapularis, possibly representing the insertion of the fused pectoralis minor. Meckel-j**, in his description of a small muscle underneath the pectoralis major in the Marmot &c, says, " II est possible, au reste, * P. Z. S. 1832, p. 74. t Loc. cit. p. 251. |