616 DR. ROLLESTON ON THE HOMOLOGIES OF CERTAIN 



coid membrane which separates the " subclavius" of the mammal from the insertion of the 

 internal intercostals, the serial homologues of the deep layer of the subclavius of the bird. 



Secondly, the nerve which supplies the pectoralis secundus of the bird is obviously 

 homologous with the nerve which supplies the subclavius of the mammal, and not with 

 the nerves which supply the greater and lesser pectorals in that class. The nerve 

 to the bird's pectoralis secundus comes, in the Accipiter Nisus, from the anterior factors 

 and lower aspect of the brachial plexus; and the nerve corresponding to the ex- 

 ternal respiratory of Bell comes off from the dorsal surface of the same part of the 

 plexus, just as the subclavius and external respiratory do in Man, whilst the nerve 

 to the great depressor pectoral muscle comes off lower down, and does not send any 

 branch to the pectoralis secundus, which, if it were the homologue of the pectoralis 

 minor, it would do. In the Sparrow-Hawk the trunk formed by the first and second 

 of the cervical nerves, which join to form the plexus, gives off nerves to the sub- 

 scapularis, the teres major, the subclavius, and the pectoralis secundus. The nerve to 

 the last muscle is the first one given off from the front of the plexus, and it gives no 

 branch to any other muscle. The external respiratory nerve, which passes down behind 

 the brachial plexus and between the serrati antici and the ribs, has one root from the 

 second factor of the plexus and one from the third. Its origin, therefore, and distribu- 

 tion, and its relation to the nerve to the pectoralis secundus, are close enough in resem- 

 blance to those of the human external respiratory to leave no reasonable doubt as to its 

 hoinological identity with that nerve, and, by consequence, as to tlte identity of the nerve 

 to the avian pectoralis secundus with the nerve to the mammalian subclavius. The 

 nerve to the pectoralis major of the Sparrow-Hawk is given off from a point lower down 

 in the plexus; and the three main trunks, which mainly make up the plexus, seem, as in 

 the Alligator, all to give factors to it. It can scarcely be doubted that this nerve is the 

 homologue of the two anterior thoracic nerves of the mammal. 



If, thirdly, we are asked, Where is the second pectoral of the mammal to which 

 the anterior thoracie nerves supply fibres as well as to the pector.ilis ? the answer 

 furnishes us with our third argument, and runs thus :— In the youn g Sparrow-Hawk 

 it is easy to divide the pectoral mass into two strata, the upper and deeper of which 

 is much the feebler, but which is continued into a tendon continuous, indeed, with 

 that of the pectoralis magnus on its deep surface, but prolonged upward, from its 

 primary insertion into the inner tuberosity, very distinctly to the coracoid, bein 

 loosely connected with the tendons of the biceps, and ultimately attached (which the 

 tendon of the biceps is not) to the precoracoid epiphysis of the clavicle. The descriptior 

 which I have given of the posterior layers of the great pectoral of the young bird is not 

 as far as I know, verifiable in the muscle, though it is in the tendon, of the adult 



O 



But it corresponds very closely with a description of the posterior layers of the great 

 pectoral m the Common Rat, Mns clecumanm ; and it is d i fflcult to see ho w it is possible 

 to deny the homology of the upper fascicles of the posterior layer of the great pectoral 

 in this ammal with the pectoralis minor of Man. The nerve-trunk which thus supplies 

 in the bird what I hold to be the homologue of the two pectoral* of anthropotomy 

 does it .s true, give off a nerve to one other muscle, viz. the coraco-brachializ inferior. 



