REPORT ON THE MARSUPIALIA. i 



Pectoral muscles. — These muscles present a very complicated arrangement. In the 

 discus there are four distinct factors, viz. : — 



or. 



(1) A large superficial fan-shaped muscle, ] 



(2) An anterior deep band of muscular fibres, J 



(3) A posterior deep muscle, probably the pectoralis minor. 



(4) The pectoralis quartus. 



The first two may be regarded as being constituent parts of the pectoralis major. The 

 large superficial fan-shaped portion (PL II. figs. 1 and 4, p l .) arises from the whole length 

 of the sternum and ensiform cartdage. From this it passes outwards, and is inserted 

 into the outer lip of the bicipital groove, where it coalesces slightly with the clavicular 

 deltoid. The deep portion (PI. II. figs. 1 and 4, jf.) takes origin from the anterior third 

 of the sternum, and throughout is quite separated from the preceding muscle. Stretching 

 outwards and slightly forwards as a thick fleshy band, it is inserted into the externa! 

 tuberosity, and a small portion of the external bicipital ridge of the humerus. The 

 pectoralis minor (PI. II. figs. 1 and 4, p 3 .) springs from the posterior two-thirds of the 

 body of the sternum, and also from one or two of the costal cartilages. From this it 

 proceeds outwards and forwards, and sinking under cover of the small deep portion of 

 the pectoralis major, it is inserted into the inner margin of the great tuberosity of the 

 humerus, into the capsule of the shoulder-joint, and through the medium of a strong 

 aponeurotic membrane into the tendon of the supraspinatus and the coracoid process. 

 Its origin from the costal arches and its indirect insertion into the coracoid process seem 

 to indicate that this muscle is the pectoralis minor. The pectoralis quartus (so-called by 

 Professor Macalister) (PI. II. figs. 1 and 4, p 4 .) arises behind the ensiform cartilage from 

 the linea alba, and from the fascia over the rectus abdominis, and extends forwards and 

 outwards under cover of the superficial part of the great pectoral to the upper part of 

 the pectoral ridge into which it is inserted. The humeral portion of the panniculus 

 carnosus (PI. II. fig. 1, p.c.) fuses with it at its insertion. 



In the Phascogale the pectoral muscles are identical with those of the Cuscus with 

 two exceptions, viz., (1) there is no deep muscle corresponding with the deep portion of 

 the pectoralis major ; (2) the pectoralis quartus is more strongly developed, and is 

 more in the form of a fleshy band. 



In the Tliylacine the pectoral muscles consist of a small superficial muscle, a large 

 deep muscle, and a very rudimentary pectoralis quartus. 



The superficial muscle is the representative of the pectoralis major. It arises from 

 that part of the sternum which lies in front of the junction of the second costal cartdage. 



The deep muscle, which is the representative of the pectoralis minor, springs from 

 the whole of the sternum behind the preceding muscle, and also from some of the 

 posterior costal cartilages. 



