124 ON THE HOMOLOGIES OF CEETATN MUSCLES 



which, while it coincides in insertion, differs little in origin from 

 the epicoraco-humeral of the crocodile, except in the possession of 

 a small head from the middle point of the sternum. The essential 

 relations held by the muscles lettered e h in each of the two figures 

 3 and 4, to the surrounding parts, seem to be much the same in 

 the intervals between their origin and insertions. The crocodilian 

 muscle, however, receives a head from the visceral surface of the 

 scapula, in compensation for the absence of the fibres which in the 

 emu arise from the coraco-clavicular membrane and sternum, but 

 which in the reptile appear to have been displaced by its great 

 pectoral from its less-developed sternum. The pectoralis secundus 

 of the sparrow-hawk, as already described, receives in a similar way 

 an accession of fibres from the visceral surface of the coracoid and 

 sternum. It is possible that the fascicle ei in the crocodile, if the 

 homologue of a dislocated anterior segment of rectus abdominis, 

 may foreshadow the sternal prolongation of the avian levator 

 humeri. But though this muscle seems exceedingly ready to build 

 up extraneous elements into its own mass, it is perhaps more easy, 

 with our knowledge of this muscle, as it is to be described hereafter 

 in the frog, to conceive the epicoraco-humeral as encroaching from 

 its own area on to that of the sternum, than to conceive of it as 

 fusing with a * rectus thoracis,' even though the subclavius of the 

 wombat does actually do this. 



Another point of difference is presented by the retention in 

 the emu of a rudiment of the coracoidal pulley, which is not 

 indicated in the crocodilian shoulder-girdle, either as a nascent 

 or as a retrograding structure. The pectoralis secundus of birds 

 does not by any means invariably receive fibres from the cora- 

 coid ; it does receive such an accession in the sparrow-hawk, but 

 it does not do so in the pigeon (Columba livia) for example ; 

 it may (as seen in great simplicity in the emu and apteryx) 

 have a mesoscapular factor actually joined to its tendon, and 

 inserted so as to be one in function with it, and it may be 

 considered, therefore, to be essentially a sterno-praecoraco- scapular 

 muscle. The sternal factor is lost in the ostrich {Struthio camelus), 

 and in the rhea {Rhea americana). Similarly the muscle figured at 

 e h, fig. 3, from the crocodile, is simply a ' praecoraco-scapularis,' 

 with one head arising from the anterior surface of the expanse 

 of bone constituted by the confluence of scapula and coracoid in 



