RELATION OF DIFFERENTIATION TO FUNCTION 157 



limb attracts the third, fourth, and fifth nerves. The transplanted limb some- 

 times attracts. the sixth, seventh, and eighth nerves, but in addition it may 

 receive a branch of the fifth nerve. We have already pointed out that a limb 

 supplied by the sixth, seventh, and eighth nerves does not function properly. 

 In this case, however, with only a branch of the fifth nerve entering the 

 transplanted limb, function is perfect. Further experiments show that any one 

 of the third, fourth, and fifth nerves is sufficient to give perfect function to a 

 forelimb. Thus any difference in the organization of these three nerves must 

 be labile or subject to change by experimental procedure. 



When two limbs are each innervated by the fifth nerve, as in Figure 92, 

 they show a very striking phenomenon. When the normal limb contracts, the 

 transplanted limb contracts simultaneously. If a muscle in one of the two 

 limbs is stimulated, the same muscle in the other limb will contract simulta- 

 neously. This phenomenon has been called homologous junction. It indicates 

 that a muscle responds to a specific kind of stimulus regardless of the nerve 

 which is supplying the muscle. 



Relation of differentiation to function 



Let us now consider the relation of function to the morphological and 

 physiological development of the nervous system. As an animal develops, its 

 muscular responses form a pattern which becomes more and more complex 

 as the nervous system develops. For example, in the amphibian the first 

 contractions occur on one side of an animal, bending it into a C shape 

 (Fig. 93). With further development contractions occur on both sides simul- 

 taneously, giving an S-shaped configuration to the animal. A number of 

 contractions following this S-shaped pattern results in the normal swimming 

 movements in the amphibian larva. Does the animal have to learn this pattern, 

 or does normal development of the nervous system occur independently of 

 the function ? 



Experiments in which animals have been anesthetized during this so-called 



Fig. 93. First reflexes in am- 

 phibian larva. C-shaped larva result 

 from contraction of muscles on 

 right side. Then muscles contract on 

 left side. Finally, some muscles con- 

 tract on both sides, imparting an S 

 shape to the larva. C -shaped S-SHAPED 



