i'2 Proceedings of the Royal Physical Society, 



(2) into the middle of the humeral shaft below the teres 

 major; (3) into the internal condyle of the humerus. Ac- 

 cordingly, these parts are distinguished from each other as 

 G, hrevis, C. medius, C. longus. The path of the musculo- 

 cutaneous nerve downwards into the arm is between the G. 

 medius and the G. longus. Now, it occasionally happens that 

 all these parts are found in the human arm, and this year I 

 have seen them all present and well developed in the two 

 arms of the same subject ; but in man the common arrange- 

 ment is for G. hrevis to be entirely absent, while G. longus 

 has retreated from the internal condyle, and has taken a 

 fresh attachment to the internal supracondyloid septum and 

 the humeral shaft close against the G. medius. As a result, 

 a single muscle is produced, through which the musculo- 

 cutaneous nerve passes, and this is described as the normal 

 condition in man. We have here, therefore, not only a good 

 example of muscular evolution, but also a case of reduction 

 in the number of muscles. 



In the forearm and hand, the special development of the 

 prehensile function, and of the precise co-ordinated move- 

 ments of the digits, afford frequent opportunity for the pro- 

 gressive evolution of the muscles concerned. Two of these, 

 viz., the brachialis aiiticus and the siqnnator longus radii, 

 which lie close to each other above the elbow-joint, are 

 interesting in connection with the importance of nerve 

 supply, as a guide to correct homology. Each of these 

 muscles is primarily a flexor of the forearm. The former is 

 innervated from the musculo-cutaneous nerve, and is inserted 

 into the coronoid process of the ulna ; the latter is innervated 

 from the musculo-spiral nerve, and is inserted into the base 

 of the styloid process of the radius. By a transference of a 

 portion of the supinator muscle to, and its insertion with, 

 the hrachialis anticus muscle into the coronoid process of the 

 ulna, the latter is augmented and its flexor action increased, 

 and so constant is this condition that the hrachialis anticus 

 of man is always described as possessing two nerves of 

 supply, viz., the musculo-cutaneous and the musculo-spiral. 

 An interesting form of progressive evolution is found in the 

 supinator radii hrevis muscle. Among anthropoid apes this 



