ANIMAL MECHANICS. 



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do not undergo the same fusion as in the forearm, or as their 

 tendons undergo. Thus, we always find the fl. hall. long, to 

 be a muscle quite distinct from the fl. dig. long., even when 

 they act by a united tendon ; and, on the contrary, in the 

 forearm when the tendons of the fl. 'poll. long, and fl. dig. prof. 

 become united, we generally observe these muscles to become 

 united also. This difference in the muscles of the forearm 

 and leg, I believe to be due to a mechanical necessity. In 

 the propulsion of the body forwards by means of the hind 

 foot, it is necessary to press the inner toe (hallux) outwards 

 and backwards against the ground ; a motion that is effected 

 by the separate action of the fl. hall. long, muscle, and ma- 

 terially aided by the action of the peroncem longus muscle, 

 which lifts upwards and inwards the outer side of the near end 

 of the foot. Such an action of the fore foot is seldom neces- 

 sary or desirable, and hence the differentiation of they?, poll, 

 long., when it exists, is made to increase the grasping or 

 hand-like action of the thumb, and not for the purpose of pro- 

 gression ; and is, therefore, found to be less constant in its 

 occurrence in various animals than the flexor hallucis longus. 



(a). Man. — The differentiation of the thumb from the 

 other fingers is carried farther in man than in any other 

 animal, because in man the appropriation of the fore foot to 

 the use of the brain is carried farther. The fl. poll. long, is 

 quite distinct from the fl. dig. prof., and in the latter muscle, 

 the tendon of the index finger is usually distinct from the 

 tendons of the third, fourth, and fifth fingers, far above the 

 wrist. It is a curious fact that in the apes, that in other re- 

 spects most nearly resemble man, the differentiation of the 

 action of the thumb is most imperfect. Thus, in the Chim- 

 panzee (Troglodytes Aubryi), Professor Gratiolet found that 

 the thumb is bent by an oblique slip of the tendon of the 

 muscle that bends the other fingers ; it is, therefore, influenced 

 by the common movements of flexion, and consequently not 



