MUSCULAR SYSTEM OF MAMMALIA. 11 



the back of the pouch. If the anterior fascicles of the diverging 

 and embracing fibres be dissected from the posterior ones, the 

 appearance of the cremaster dividing into two layers is produced. 



The principal modifications of the muscles of the pectoral ex- 

 tremity are here described as they exist in the Perameles lagotis. 



The trapezius has its origin extended from the skull, along the 

 cervical and dorsal spines, to the fascia covering the lumbar por- 

 tion of the latissimus dorsi : its fibres converge to be inserted along 

 the spine of the scapula, the anterior ones being directly continued 

 into the pectoralis major, whereby it becomes an extensor of the 

 humerus and a protractor of the fore extremity. 



The latissimus dorsi arises chiefly from the broad aponeurosis 

 covering the muscles of the lumbar region of the spine, and from 

 the spines of the six posterior dorsal vertebras ; the fibres gradually 

 converge, the muscle increasing in thickness as it diminishes in 

 breadth, and terminating in a strong flattened tendon one inch 

 before its insertion at the upper third of the humerus. It is con- 

 nected, as in most brutes, up to and including the Gorilla, with an 

 accessory extensor (omo-anconeus) l of the antibrachium. This ex- 

 tensor takes its principal origin by fleshy fibres from the terminal 

 half inch of the fleshy part of the latissimus dorsi, and continues 

 fleshy, slightly diminishing in size to its insertion at the apex of 

 the olecranon. To remedy the inconvenience of an origin from a 

 yielding and flexible part, a thin aponeurotic slip, in Perameles, 

 attaches a part of the base of the superadded muscle and the cor- 

 responding portion of the latissimus dorsi to the sheath of the teres 

 major, and to the inferior costa of the scapula near its posterior 

 angle. The supraspinatus, a strong penniform muscle, exceeds 

 the infraspinatus in breadth by as much as the supra-spinal fossa 

 is broader than the infra-spinal one : it has a broad and strong 

 insertion into the great outer tuberosity of the humerus. The 

 infraspinatus is inserted into the upper and posterior part of that 

 tuberosity. The deltoides is a comparatively small muscle ; it 

 arises from the anterior half of the spine of the scapula and from 

 a fine aponeurosis covering the infraspinatus ; its fibres converge 

 to be inserted in the upper part of the deltoid ridge. A thin 

 small strip of muscle arises from about the middle of the 

 inferior costa of the scapula, beneath the infraspinatus ; its 

 fibres pass forward and join the lower margin of the small del- 

 toid, thus bracing and enclosing the tendon of the infraspinatus. 



1 lxxx'. p. 289 (1846): the muscle is termed ' dorso-epitrochlien' by Duvernoy in 

 the Gorilla, i". p. 80 (1855), where it is inserted into the inner condyle of the 

 humerus. 



