146 PROCEEDINGS OF THE ACADEMY OF [1888. 



has become segmented off as a distinct band, which consequently 

 forms the posterior boundary of the p. major and the anterior border 

 of its involuted fold. 



Such an explanation would derive probability from the mechan- 

 ical relations of the parts. As the original pectoral mass travelled 

 backward, and its posterior border came, as in this specimen, to lie 

 more nearly in a longitudinal than in a transverse line, it would 

 encroach more and more upon the axilla, that is, it would deepen 

 the anterior axillary fold. As this fold began to project from the 

 general contour of the body, there would be a tendency, especially 

 during anterior extension of the arm, for it to be repressed by fold- 

 ing in, and the backward direction of this fold would be determined 

 by the general curve of the integument. Such involutions of the 

 posterior border of the pectoralis major occur in many mammals, 

 and often, as in the human subject, in a form which makes it ap- 

 pear improbable that they can have been the result of fusion. This 

 view is also favored in the present subject by the course of the deep 

 anterior thoracic nerve, which, as mentioned above, runs completely 

 around the concavity of the fold. 



Through the kindness of Dr. Harrison Allen, I was enabled to 

 examine the pectoral in a young Ursus americanus. In this spec- 

 imen the first striking character was the much greater thickness and 

 power of the entire mass. The concavity of the fold was partially 

 obliterated by narrow fasciculi extending between its walls, but the 

 type of the polar l)ear Avas still easily seen ; the ventro-humeral slip 

 wa.s much stouter and separated from the rest of the muscle by a 

 ver}' distinct line. The p. minor was as in U. maritimus; of the two 

 remaining divisions, the more posterior was so intimately fused with 

 the p. major, as to be scarcely distinguishable. The fasciculi run- 

 ning from inside the fold to the sternum, began to give the idea of a 

 separate muscle lying immediately behind the pectoralis minor, in 

 the same plane. The thoracic nerve ran about as before. 



In an alcoholic, probably new born, specimen of Melursus libyvs 

 in the collection of the Academy of Natural Sciences of Philadel- 

 phia, the modification of these parts had gone so far that they closely 

 resembled the condition in the Cat, though still considerably simpler 

 than in that animal. The most superficial division was a nari'ow 

 ribbon (evidently the "first division of the pectoral" of Mivart,* or 

 the cephalic " pecto-antebrachialis " of Wilder and Gage') rising 



1 The Cat. 



' Anatomical Technology. 



