276 SHOULDER-GIRDLE IN MONOTREMES, 



surface of the 2nd-6th rib (inclusive), which lies internal to the 

 rectus; and also from 1*5 cm. of the anterior portion of the linea 

 alba and the adjacent aponeurosis of the external oblique. 



Insertion. The whole muscle is divided into anterior and 

 posterior divisions by a tendinous intersection (Fig. 5, Ti.) which 

 reaches from the presternum to a point situated a short distance 

 from the proximal extremity of the pectoro-deltoid ridge. The 

 anterior fibres of the anterior division run outwards, and some- 

 what posterior, to be attached directly to the ventral portion ^of 

 the external border of the great tuberosity. The more posterior 

 fibres of the anterior division, however, run to be inserted along 

 the tendinous intersection, as do the anterior fibres of the posterior 

 division, while the most posterior fibres run forwards and outwards, 

 some to be attached by tendon to the proximal two-fifths of the 

 pectoro-deltoid ridge, others to coalesce with the deep layer of the 

 panniculus, as above described. 



Relations. The muscle is separated from its fellow by a 

 well-marked median line throughout. The antero-external border 

 of the anterior division of the muscle is closely related to the 

 clavicular deltoid (Fig. 5, Dlt. C), but no difiiculty is experienced 

 in differentiating the one from the other in well preserved speci- 

 mens ; the dividing line being a cellular interval quite apparent 

 when the superficial and deep fasciae of the region are removed. 

 The border of the clavicular deltoid does not, however, distinctly 

 overlap the pectoral as in Echidna. The few fibres which arise 

 from the transverse part of the interclavicle are closely related to 

 the inner fibres of origin of the episterno-cleido-mastoid. The 

 postero-external border of the muscle is overlapped by the P. 

 quartus. The muscle hides from view the same muscles as those 

 mentioned under Echidna. 



Pectoralis quartus. (Fig. 5, Pt. Q.). 

 Ornithorhynchus. 

 Origin. This muscle is triangular in outline, and arises from 

 the anterior half of the linea alba (with the exception of the 



