612 DR. ROLLESTON ON THE HOMOLOGIES OF CERTAIN 



the pra-sternal proosteon and from the cartilage of the first rib, exteriorly to which latter 

 point of origin the epicoracoid has coalesced with the vertebral rib. It passes up behind 

 the osseo-cartilagino-fibrous bar made up of ligaments, prgecoracoid, clavicle, and meso- 

 scapular segment (see Parker, I. c. pi. 24. fig. 6) and reaching from the apex of the 

 anterior prolongation of the sternum to the acromion and coracoid. Some of its fibres 

 are inserted into the coraco-clavicular ligament, and some, though fewer, into the clavicle 

 itself. But from the clavicle it receives a large accession of muscular fibres in return ; 

 and these, joining with the fibres of the original muscle, pass on to be inserted along the 

 anterior edge of the acromion, and also to become continuous along the mesial or down- 



ward-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 

 mammalian 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 mor- 

 phologically equivalent to the pector alls secundus or levator humeri of the bird. A larger 

 muscle, the larger " sternoscapular " of Mivart and Murie, arises from the mesosternum 

 in the Guineapig, and takes a similar course to that of the " 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 "subclavius" here has an origin, as it has also in the Crested 

 Agouti, Dasyprocta cristata (Mivart and Murie, /. c. p. 398), and in other mammals, from 

 a sternal as well as from a costal element in immediate connexion with the epicoracoid, 

 and, indeed, also with the praecoracoid, we may think ourselves just ified in regarding the 

 muscle with these points of origin as the morphological equivalent, as to its origin at 

 least, of the entire pectora lis secundus of the bird, independently of the "greater sterno- 

 scapular" muscle. But, inasmuch as the pector alls secundus of the bird shows some 

 tendency to self-mult iplication, as seen in the suborder Gallinse, it may be well to con- 

 sider the two sternoscapular muscles as, either severally or fused, homologous * ith either 

 two distinct pectorales secundi or a single one. The rectus abdominis is prolonged up to 

 the first rib, and is overlapped in the Guineapig by a muscle homologous with that figured 

 at e o in fig. 3, of the Crocodile's shoulder-joint muscles ; but it does not give any tend i nous 

 or muscular factor to the subclavius here, as it does in the Wombat {Fhascolomys wombat). 

 A description of the arrangement of these parts in this latter animal will be found in mv 

 description of fig. 3, which, though taken from a dissection of a Crocodile, makes the 

 account of the structures, as seen in the Wombat, much more intelligible. Both descrip- 

 tions alike warn us not to lose sight of the possibility that the avian levator humeri may 

 have borrowed a factor from the anterior prolongation, over the sternum, of the rectus 

 abdominis. The arrangement of these selfsame structures in the Aardvark (Orycteropus 

 Capensis) enforce the same lesson. In this animal (a detailed account of the myology of 

 which by J. C. Galton, Esq., appears in the Linnean Society's 'Transactions,' vol. xxvi. 

 p. 571) the subclavius arises from the manubrium, from the cartilage of the first rib, and 

















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