226 HISTORY OF THE HUMAN BODY 



of the intermediate slips. Aside from these a second series of 

 slips, more superficial than the above, appears in the higher 

 amphibians, and these develop in mammals into the rhomboid- 

 ens system. This inserts into the scapula and consists pri- 

 marily of a slip from the occipital bone, rhomboideus capitis, 

 and one from the vertebral spines in the interscapular region, 

 rhomboideus dorsi. Both of these occur in most mammals, 

 but in man rhomboideus dorsi alone is normally present, sub- 

 divided into two slips, major and minor, while rhomboideus 

 capitis appears only as a rare anomaly. The pectoralis in 

 many mammals forms a complex system of distinct and semi- 

 distinct portions, showing at least a superficial and . a deep 

 layer. In man and the anthropoids these two layers are repre- 

 sented by two muscles, pectoralis major and minor respectively. 

 The subclavius is a differentiation from the deeper layer. 



Of the intrinsic group the dorsalis scapula becomes mainly 

 the deltoid, often divided into several portions, spino-deltoid, 

 acromio-deltoid, etc., but single in man. A small portion of 

 this muscle becomes the teres minor, topographically associated 

 with the teres major, derived from the latissimus. The supra- 

 coracoideus is probably represented by the supra- and infra- 

 spinati, which have extended dorsally over the scapula, pushing 

 their way beneath the deltoid, as this muscle has gradually 

 lifted itself up from the general outer surface of that bone. 

 The pro cor aco-humer alis seems to have become lost, together 

 with the axial slips that insert into the procoracoid. 



The anconeus, the extensor muscle of the upper arm, varies 

 mainly in the number and position of its heads, and not in its 

 insertion or general position. Its identity with the human 

 triceps has been already commented on. On the flexor side of 

 the upper arm the history is not as plain. In the mammals 

 there are two long muscles that insert into the forearm, the 

 biceps brachii, that arises from the shoulder girdle and inserts 

 by a tendon into the proximal portion of the radius, and the 

 brachialis [anticus] that arises along the shaft of the humerus, 

 and inserts into the proximal end of the ulna. Aside from 

 these, there is a coraco brachialis, from the coracoid process 



