1888.] NATURAL SCIENCES OF PHILADELPHIA. 145' 



the two anterior thoracic nerves, which embrace and supply it much 

 as in the human subject. 



In most mammals there are behind the pectoralis minor, certain 

 other muscular elements — " pectoralis quartus " " xiphi-sterno-hu- 

 meral, " " ventro-humeral, " etc. — wljich have been the subject of 

 much discussion. Wilder, as mentioned above, considers most of 

 these as parts of the p. minor, but the most posterior slips he speaks 

 of ^ as probably " differentiated portions of the main pectoral mass. " 

 In the Carnivora, Macalister^ alludes to the "fourth pectoral or 

 brachio-lateral part of the panniculus." As regards the Marsupialia, 

 Cunningham^ remarks : " There is a diversity of opinion regarding 

 the character of the pectoralis quartus. Owen looks upon it as ' a 

 dismemberment of the pectoralis major.' Humphrey and Macalis- 

 ter believe that it is in an ' intermediate piece of the superficial ex- 

 ternal muscular sheet between the pectoralis major and latissimu& 

 dorsi. ' Its close connection in many cases with the panniculus 

 carnosus would almost seem to indicate that it is merely a portion of 

 this muscle. In Cuscus, indeed, it appeared to be simply the thick- 

 ened lower margin of the panniculus, the connection between them 

 is so intimate. " 



It appears to me almost unquestionable that, as implied in the 

 above statement, the posterior slips of the more deeply inserting 

 pectoral mass are phylogenetically different from the more anterioi' 

 portion of the same layer. The opinions just quoted as to 

 the derivation from panniculus or latissimus, may each of them be 

 correct in different types, but the structui'e of the present specimen 

 suggested to me an explanation, a development of the idea of Owen, 

 which seems to be sustained by the series of Carnivora, and which 

 would well explain the oblique position of these posterior elements. 



According to this view, the structure in the polar bear would in- 

 dicate that an originally simple and unsegmented pectoralis major 

 muscle has gradually encroached backward upon the abdomen ; that 

 during this process its axillary border has become folded under and 

 acquired a retrogressive attachment to the humerus beneath the 

 parent mass ; that finally, the entire posterior border of the muscle 



1 Loc. Cit. 



^ Muscular Anatomy of the Civet and Tayra. Proc. Roy. Irish Acad. 



Vol. I, Ser. 2, p. 508. 

 3 Report on the Marsupialia. Challenger Rep. Zool. Vol. V, p. 8. 



