THE MUSCLES OF THE FORE-LIMB 35 



as is the general rule, by the segmentation of muscular 

 sheets into separate muscles; and a comparison may be 

 made directly between this simple type and the more 

 specialized condition seen in some other animals. The 

 superficial portion of the proximal mass becomes the 

 humeral head of the M. pronator radii teres. The deep 

 portion of this mass, which is blended with the inter- 

 mediate sheet, becomes the M. pronator intermedius, 

 or ulnar head of the M. pronator radii teres. The distal 

 mass, or M. pronator manus, looses its insertion to the 

 third segment in many forms, and, united with the lower 

 portion of the intermediate sheet, it constitutes the 

 M. pronator quadratus. 



With the differentiation of the muscular sheets of 

 Cryptobranchus there is therefore afforded a myological 

 basis for (1) a bicipital M. pronator radii teres, and (2) a 

 definite M. pronator quadratus, which may, or may not, 

 possess extensions downwards to the carpus. 



This is the condition found in many unspecialized 

 Reptiles and Amphibians, as well as in the most primitive 

 Mammals, and it may be regarded as the simplest type 

 of resolution of the primitive muscle sheets. 



But if this condition of the pronator group may be 

 regarded as primitive, it is obviously one that is very 

 readily departed from at the dictates of functional 

 demands for the stability of the limb, and one that has 

 a very limited representation among the existing 

 mammalian types. 



To gain an insight into the possible modifications of 

 this muscle group it will be best to take each muscle 

 separately; and the most simple comparison may be 

 made in the apparently illogical order of taking the 

 human condition first. 



The M. pronator radii teres is a typical bicipital muscle 

 in Man. The superficial, and larger, portion arises from 

 the medial aspect of the lower end of the humerus; the 

 deeper, and smaller, portion is derived by a tendon from 



