

614 DR. ROLLESTON ON THE HOMOLOGIES OF CERTAIN 



humeri, all that is necessary is the provision for its tendon of some snch pulley-like 

 arrangement by the coracoid. What I have to show is that, as a matter of fact, the 

 muscle which, in the economy of nature, has been worked up into the avian levator 

 humeri is one and not the other of two, either of which, so far as we can see, might have 



been so employed. 

 I cannot adopt Prof. Pagenstecher's views (Zool. Gart. April 1867, p. 125), accordin 



to which the two mammalian pectorales are referred to two entirely distinct sets of 

 muscles, the pectoralis major being one of a ventral, and the pectoralis minor one of a 

 dorsal series. The single fact of their both receiving a nerve-supply from the same nerve 

 (the internal anterior thoracic) seems to be conclusive against the validity of this suggestion. 

 And I believe further that it is impossible to observe how the posterior fascicles of the 

 pectoralis major in the lower mammalia have their tendons prolonged up from the lesser 

 tuberosity of the humerus and the bicipital groove, which is bridged over by pectoral 

 tendons, to the coracoid process without being convinced that the pectoralis minor of 

 anthropotomy is but a specialized anterior fascicle of the deeper portions of the pecto- 

 ralis major. The commonest variation, according to Mr. Macalister (Cambridge Journal 

 of Anatomy and Physiology, May 1867, p. 317), which the pectoralis minor exhibits in 

 Man is an insertion of the lowest part of its tendon, missing as it were the coracoid 

 process, into the coraco-brachialis muscle. 



The arguments which I shall now adduce to show that the pectoralis miaor is not 

 the homologue of the avian pectoralis secundus will at the same time go some way 

 towards proving that the primary insertion of the mammalian muscle is a humeral 

 one, more or less internally to the tendon of the superficial layers of the greater 

 pectoral, whilst the primary insertion of the avian is also a humeral one, but one more 

 or less externally placed to that of the great pectoral depressor of the humerus. The 

 attachment of the mammalian pectoral to the coracoid is secondary in its history ; and 

 the like place in the series of its specializations is held by the development of the coracoid 

 pulley in the history of the avian pectoralis secundus. The insertion of the " subclavius" 

 into the clavicle and the junction of the "pectoralis minor" with the supraspinatus 

 will be seen in the course of the argument to be exclusively mammalian developments. 

 Now there are three lines of argument for showing the homological identity or non- 

 identity of any two muscles. The first line of argument shows that they hold the same 

 or different relations to the other structures they are connected with at their origin, in 

 their course, or at their insertion. The second shows that they are or are not supplied 



by the same 



The third shows that either in their adult condition 



the condition of development they are independent or connate. A comparison of the 

 pectoralis minor of the mammal with the pectoralis secundus of the bird furnishes us, 

 under each of these three heads, with an argument for their non-identity. Under the 

 first head I would observe, as Prof. Pagenstecher has already been quoted as doing, that 

 the pectoralis minor of the mammal lies to the outer side of the costocoracoid mem- 

 brane, to the inner side of the homological representative of which structure, viz. the 

 coracoid, the pectoralis secundus of the bird passes. I do not mean to deny, nor yet to 

 affirm, that the pectoralis minor of the mammal may not be one of the same series of 



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