1888.] NATURAL SCIENCES OF PHILADELPHIA. 147 



from the manubrium and running obliquely ontward and backward 

 to blend with the eephalo-humeral and insert in common with it on 

 the middle of the humerus. Beneath this and crossed oblicjuely by 

 it was a large square mass (" ectopectoralis " of Wilder ; " second 

 division" of Mivart) which rose along somewhat less than the anterior 

 half of the sternum and inserted into the proximal half of the 

 humerus. 



Upon reflecting this, three deep divisions were exposed. The 

 most antei'ioi", p. minor, rose on the middle third of the sternum, 

 and inserted as in the bear ; it would evidently correspond to that 

 section in the cat, which is termed by Mivart the " third division, " 

 and by Wilder the " entopectoralis, div. cephalica. " The next di- 

 vision, which immediately adjoined the last, rose along the remainder 

 of the sternum and posterior costal cartilages and inserted by apon- 

 eurosis into the pectoral ridge of the humerus at the middle of the 

 insertion of the " ectopectoral. " It occupied the position of the 

 "entopectoralis, div. caudalis" of Wilder and seems to correspond 

 to the pectoralis quartus of Marsupials. The thoracic nerve, coming 

 out behind the p. minor, ran backwards a short distance over the 

 surface of this muscle and then dipped into its substance where it 

 could be traced between two ill-defined laminae almost to its posterior 

 border. This muscle seems to be the realization of the tendency 

 which was showing itself in the two other bears. The position of the 

 mass, its partial separation into two laminae, the relation to these of 

 the thoracic nerve, and a certain greater obliquity of its antero- 

 external than its antero-internal fibres, all appear to indicate that 

 it is the representative of the involuted fold of the pectoralis major 

 of the polar bear, which has been split off from the main mass and 

 had its two layers nearly combined. According to the provisional 

 hypothesis of Wilder this division would be part of the pectoralis 

 minor, but the present comparisons indicate it rather to be an inde- 

 pendent derivative of the pectoralis major. 



The remaining slip, which was quite delicate, rose for a short 

 distance along the linea alba and inserted beneath the last, which 

 also received the insertion of the panniculus and a slip from the 

 latissimus dorsi. I suppose it to be the ventro-humeral division of 

 the two bears and to answer to the " xiphi-humeral " of the cat, 

 which would then, according to this theory, owe its origin to the 

 pectoralis major and not to the latissimus or panniculus. 



