BY W. J. S. JIcKAY. 275 



by a slight interval. The P. qiiartus lies superficial to and hides 

 from view its postero-external border, while the pectoralis itself 

 covers part of the epicoraco-hunieral, biceps, coraco-brachialis and 

 rectus. 



P. QUARTUS. (Fig. 3, PL Q.). 

 Echidna. 



Origin. This muscle arises from the posterior one-third of 

 the ventral aspect of the metasternum, and from the anterior half 

 of that portion of the aponeurosis of the external oblique that lies 

 between the pubes and metasternum. Anteriorly the fibres of 

 origin are close to the median line ; more posteriorly they diverge 

 slightly from the linea alba, and spread out into a fan-shaped 

 sheet. 



Insertion. The muscle runs forward and outwards as a 

 thin layer, gradually becoming narrower (but thicker), and as it 

 apf)roaches its insertion it becomes cleft in a similar fashion to the 

 deep stratum of the panuiculus. The upper layer (Fig. 3, Pt. Q.) 

 is inserted superficial (ventral) to the P. major on the external 

 aspect of the ventral portion of the greater tuberosity ; the deeper 

 layer (Fig. 3, Pt. Q.), continuous at its outer border with the other, 

 is inserted on the greater tuberosity beneath the P. major. 



Relations. At its origin this muscle hides from view the 

 anterior fibres of the large pyramidalis which is inserted on the 

 metasternum. As it runs forward and outwards it hides portion 

 of the rectus, and the postero-external border of the P. major ; 

 and is hidden at its insertion by the superficial layer of the deep 

 part of the panniculus. 



Pectoralis major. 

 Ornithorhynchus. 



Origin. This muscle is triangular in outline and arises from 

 the ventral aspect of the transverse bar of the interclavicle at its 

 junction with the median portion of that bone ; from one-half of 

 the ventral surface of the median portion of the interclavicle, 

 presternum and mesosternum, and from that part of the ventral 



