BY W. J. S. McKAY. 281 



external head of the humeral part of the triceps. (Fig. 4, Dlt. C.) 

 Origin. (6) Scapular portion. (Fig. 10, Dlt. -S'.).— This 

 muscle arises from the external edge of the anterior third of the 

 A-ertebral border of the scapula, and from the upper third of the 

 external edge of the spine of the scapula. 



Insertion. The muscle, long and slender, runs downwards, 

 outwards and posterior, to be inserted, by a narrow well developed 

 tendon, on the posterior surface of the humerus in a depression 

 close to the distal one-third of the pectoro-deltoid ridge ; the 

 tendon being surrounded, as mentioned above, by the horse-shoe- 

 shaped insertion of the clavicular part. 



Relations. The muscle is partly hidden at its origin by 

 the insertion of the posterior portion of the trapezius ; while it 

 overlies the subscapularis (very slightly) and the infraspinatus. 



M, DELTOIDEUS. 



Ornithorhynchus. 



Origin, {a) Acromio-clavicular j^ortion. — The muscle arises 

 from the ventral surface of the transverse portion of the inter- 

 clavicle throughout its length (excepting the small area adjoining 

 the median portion from which the P. major derives a few fibres 

 of origin) ; and from the ventro-external surface of the acromion. 

 (Fig. 5, Dlt. G.J. 



Insertion. The muscle, in outline a parallelogram, runs 

 outwards and posterior. On approaching its insertion it is cleft 

 from without inwards into a superficial and deep layer. The 

 superficial and larger division is in.serted fleshy on the distal three- 

 fourths of the pectoro-deltoid ridge ; the deep division is inserted 

 parallel with, and just internal to, the proximal half of the upper 

 division, on the posterior face of the humerus ; a tubercle on which 

 the scapular deltoid is inserted intervening between the two 

 divisions. {Dlt. C. x. Fig. 8.) 



Relations. The fleshy fibres of origin are intimately 

 related, internally, to the episterno-cleido-mastoid, while externally 



