122 ON THE HOMOLOGIES OF CERTAIN MUSCLES 



and the nerve corresponding to the external 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 subscapularis, 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 distri- 

 bution, and its relation to the nerve to the pectoralis secundus, are 

 close enough in resemblance to those of the human external 

 respiratory to leave no reasonable doubt as to its homological 

 identity with that nerve, and, by consequence, as to the 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 thoracic nerves supply fibres as 

 well as to the pectoralis ? the answer furnishes us with our third 

 argument, and runs thus : In the young 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, being 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 description which I have given of the posterior 

 layers of the great pectoral of the young bird is not, as far as 





