280 SHOULDER-GIRDLE IN MONOTREMES. 



M. DELTOIDEUS. 

 Echidna : Clavicular and scap^dar portions, Mivart, Westling, Leche. 

 Ornithorhynchus : Clavicular portion, rerjarded as part of P. major, 

 Meckel, Owen, Coues, Leche, Westling, Cuvier and Laurillard ; Scapidar 

 portion, all authors. 



M. DELTOIDEUS. 



Echidna. 

 Origin, (a) Acromio-clavicular portion. This portion of the 

 deltoid arises from the ventro-anterior face of the clavicle ; from 

 the greater part of the ventral face of the lateral arm of the inter- 

 clavicle, and from the external border of the acromion. (Fig. 3, 



nu. C). 



Insertion. The muscle is quadrilateral in outline and runs 

 outwards and posterior. Narrowing as it approaches its insertion 

 it becomes cleft, in a plane parallel to the surface, into superficial 

 and deep portions which embrace the tendon of insertion of the 

 scapular portion of the muscle. The superficial layer of the 

 muscle is inserted by tendon on the postero- external border of the 

 distal two-thirds of the pectoro-deltoid ridge ; while the deeper 

 layer continuous (internally) with the superficial, has an insertion 

 (for the most part fleshy) on the posterior face of the humerus 

 close to and parallel with the distal part of the pectoro-deltoid 

 ridge. In the centre of this horse-shoe-shaped insertion is placed 

 the tendon of the scapular deltoid. 



Relations. At its origin the inner portion of the muscle 

 is hidden from view by the sterno-mastoid. The outer third of 

 the origin is placed ventral to the insertion of the trapezius on 

 the clavicle ; while the origin from the acromion lies between the 

 insertion of the trapezius on the one hand, and the origin of the 

 infraspinatus on the other. The postero-internal border of the 

 muscle is superficial to the P. major and deep part of the panni- 

 culus ; the muscle also hides in great part the epicoraco-humeral, 

 supraspinatus, and the insertion of the infraspinatus. At its 

 insertion the muscle is lelated to the origin of the supinator longus 

 and brachialis internus, and the thin tendon of origin of the 



