(520 JDR. ROLLESTON ON THE HOMOLOGIES OF CERTAIN 



Pig. 3. Muscles in connexion with the scapulo-coracoid arch and upper extremity of Crocodile (GVo- 



codilus biporcatus) . The same arrangement has been noted in Alligator lucius, with scarcely 



sternum 



any differences. 



p m. Pectoralis major. The muscle has been divided, and its origin from the 



clavicle turned mesially, and its tendon of insertion into the greater tuberosity of the humerus 

 turned outwards, p m', tendon of pectoralis major. 



eh. " Epicoraco-humeral " of Mivart (Trans. Linn. Soc. vol. xxv. 1866, p. 383), a bicipital muscle; 



its inner head winds round the coracoid face of the prseglenoid expanse, to take origin from the 

 visceral surface of the scapula ; its outer head arises from the prsecoracoid, and, to a small extent, 

 from the pnescapular portion of the prseglenoid expanse, where it is in close relation with the 

 deltoid. The inner of these two heads is intimately connected at its insertion with that of the 

 pectoralis major, and, together with the tendon from the other head, occupies on the humerus 

 a position between that of the tendon of the deltoid externally and inferiorly and that of the 

 pectoralis major internally and inferiorly. The insertion of the omohyoid corresponds with the 

 origin of the prseglenoid head of the epicoraco-humeral ; and the fibres of the two muscles are, to 

 some slight extent; continuous with each other, at least in young specimens. 



ei. Muscular fascicle which arises from second sternocostal cartilage in the same series as the 



external oblique and intercostals, but which may also be looked upon as homologous with an 

 anterior segment of the "rectus abdominis/' which is often not distinguishable from the 

 former of these muscles. It ends in a delicate tendon, which loses itself, along the coracoid 



groove, in the sternum and the origin of the pectoralis major. The rectus abdominis in many 



ordinary mammals has an insertion into the first rib (see Professor Turner, ' Cambridge Journal 



of Anatomy and Physiology/ May 1868, p. 393), from which the ordinary mammalian subclavius 



arises; and in the order Monotremata it has, in both species of Echidna and in the Ornitho- 



rhynchus, an insertion into the coracoid at no great distance from the area of origin of their 



epicoraco-humeral, which I hold to be the homologue of the subclavius of other mammals. The 



prolongation of the tendon of this muscular fascicle (ei) may be considered to be due to the 



withdrawal of the Crocodile's epicoraco-humeral to the upper end of the coracoid. Finally, as 



the rectus is often fused with the pectoral, we may regard this fascicle as a nascent pectoralis 

 minor. 



e o. Muscular fascicle ai 



cartila 



point homologous with the points of origin of the several fascicles of the external oblique), and 



forwards 



and continuous with that of the external obliqne. By contracting a close adhesion to the fibres 

 of the anterior prolongation of the rectus abdominis instead of passing freely owv them, as here, 

 this muscular slip may, in other animals, as e.g. Cynocephalus hamadryas, assume the appear- 

 ance of an outer head to this anterior prolongation. But its true character of homology with 

 the fascicles of the external oblique is seen as plainly in the Guineapig {Cavia aperia) and the 





0p&U8 



Iii the 



Wombat (Phascolomys wombat) the true relations of this fascicle are very clearly seen ; and - 

 fascicle, obviously homologous with the one lettered e i in the Crocodile, passes from under it to 

 join the subclavius arising from the first rib. This latter muscle arises, thick and flcshv, from the 



Wombat 



— — ~ «*» uuioi cuu ui inu ciavicie, ana, ov means ua •*»« 



fascia covering the supraspinatus muscle, into the whole length of the spine of the scapula. 

 Before its insertion it is joined by a fine tendon from a delicate muscular belly, arising from the 

 sixth costal cartilage, and homologous with the muscular fasciculus fi<n,ro.d here at e L Over 



Wombat 



first 



