116 ON THE HOMOLOGIES OF CERTAIN MUSCLES 



of origin the epicoracoid has coalesced with the vertebral rib. It 

 passes up behind the osseo-cartilagino-fibrous bar made up of liga- 

 ments, praecoracoid, clavicle, and mesoscapular segment (see Parker, 

 1. a, pi. 24. fig. 6) and reaching from the apex of the anterior prolon- 

 gation of the sternum to the acromion and coracoid. Some of its 



o 



fibres are inserted into the coraco-clavicular ligament, and some, 

 though fewer, into the clavicle itself. But from the clavicle it re- 

 ceives a large accession of muscular fibres in return ; and these, 

 joining with the fibres of the original muscle, pass on to be in- 

 serted along the anterior edge of the acromion, and also to become 

 continuous along the mesial or downward-looking edge of the 

 metacromion with fibres of the deltoid, which take origin from the 

 ligament prolonged inwards from the acromion to the clavicle. 

 Thus, though the greater part of the fibres of the subclavius are 

 arrested by a bony fixation into the acromion, which, it will be 

 recollected, is, in these proportions at least, an exclusively mamma- 

 lian development, it becomes, by the continuity of its innermost 

 fibres with the soft tissues placed mesially to that process, more or 

 less physiologically as well as morphologically equivalent to the 

 pectoralis secundus or levator humeri of the bird. A larger muscle, 

 the larger ' sternoscapular ' of Mivart and Murie, arises from the 

 mesosternum in the guinea-pig, and takes a similar course to that of 

 the l subclavius,' with which it interchanges fibres at the clavicle, 

 to which they both have an attachment. It is inserted, however, 

 along the upper vertebral border of the scapula ; and three-fifths of 

 the entire length of the spine of that bone intervene between its 

 insertion and that of the subclavius. If we consider that the ' sub- 

 clavius ' here has an origin, as it has also in the crested agouti, Da- 

 syprocta cristata (Mivart and Murie, 1. c. p. 398), and in other 

 mammals, from a sternal as well as from a costal element in 

 immediate connection with the epicoracoid, and, indeed, also with 

 the praecoracoid, we may think ourselves justified in regarding the 

 muscle with these points of origin as the morphological equivalent, 

 as to its origin at least, of the entire pectoralis secundus of the 

 bird, independently of the ' greater sternoscapular ' muscle. But, 

 inasmuch as the pectoralis secundus of the bird shows some tendency 

 to self-multiplication, as seen in the sub-order Gallinae, it may be 

 well to consider the two sternoscapular muscles as, either severally 

 or fused, homologous with either two distinct pectorales secundi o 





