DISCUSSION 189 



inappreciable extent. The structure of an elbow joint of 

 this character presents a number of anomalies. For one 

 teleological reason or another, many muscles take origin 

 from the humerus, instead of from one of the bones distally 

 adjacent. The position of the origins upon the epicondyles 

 makes it impossible for some of them to exert leverage upon 

 the forearm, and to all intents their functions seem to be 

 largely the same as though origin were from the forearm 

 itself. One effect of the humeral origin of these muscles 

 is to add strength to the adjoining articulation at the elbow, 

 and it may be presumed that such strengthening is not a 

 purely fortuitous result. It should be noted that when the 

 arm is relaxed, the angle of the forearm with the humerus is 

 not almost 180 degrees, as in man, but in a fresh specimen 

 after rigor mortis had passed, about 135 degrees, although 

 this angle was exceeded when considerable force was applied. 

 This position gives greater leverage to the muscles, especially 

 the flexors, of the forearm. 



The actions of the hand or fourth segment are exceedingly 

 complex, with the many bones and multitude of tendons 

 whose functions are interdependent to a high degree. Both 

 the flexors and extensors are many, though some of them 

 act as such to a relatively insignificant extent. Although 

 the wrist joint is capable of practically no rotation per se, 

 the possible rotation of the forearm coupled with the inter- 

 play between the bones of the carpus gives to the hand about 

 the same amount of sidewise movement or rotation as in 

 man. Possible supination seems to be about 90 degrees 

 from the vertical, or half that in man, while possible move- 

 ment in pronation is but a few degrees beyond the horizontal. 

 The mobility of the hand must therefore be considered as 

 only moderate in the genus Neotoma. It must be remem- 

 bered that the extensors of the hand are morphologically 

 comparable to the flexors of the foot, and the former are 

 considerably weaker than the "flexors" of the carpus. 



