THE MUSCLES OF THE FORE-LLMB 33 



tion of the muscular plan of the human arm is from that 

 seen in any lower quadrupedal form, it seems quite 

 obvious that the truly quadru^Dedal pronograde type is 

 derived from a primitive arrangement such as is retained 

 in Man. 



We have an excellent account of a very generalized 

 vertebrate type of musculature in the description given 

 by Humphry of Cryptobranchus japonicus, and to the 

 stage of evolution of the limb musculature as seen in 

 this Amphibian we shall have to make frequent reference. 

 The group of muscles which especially interests us here 

 is that which produces the rotatory movements of the 

 second segment of the limb, and in order to limit the 

 purely anatomical details we will follow the history of 

 those rotators which produce the movement of pronation 

 — i.e., turn the back of the hand upwards. This group 

 is of primary importance, since it produces that mobility 

 of the second segment which, as we have seen, is incom- 

 patible with true quadrupedal stability. The pronators 

 compose a primitive group of muscles which shows the 

 effects of alteration of function in a very definite manner; 

 the condition of the members of the group is easily 

 determined in different animal types, and their disposi- 

 tion may therefore be taken as a handy index of the 

 degree to which mobility has been sacrificed to stability 

 in the forearm. In Cryptobranchus this muscle group is 

 well developed; but, as is the case with all the muscle 

 groups of this very generalized animal, it is not so defi- 

 nitely subdivided into its component elements as it is 

 in higher forms (see Fig. 11). 



The pronators of this Am^Dhibian consist of a super- 

 ficial part, which arises from the ulnar cond^de of the 

 humerus in common with the M. flexor carpi radialis, 

 but, dissociating itself from the common mass, is inserted 

 into the lateral margin of the radius. Beneath this 

 portion is a deeper set of fibres, which at its insertion to 

 the radius becomes continuous with another sheet which 



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