168 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



out the length of the cord, diminishing in area by the way as some 

 of their fibers terminate in each segment. This system of fibers 

 is supposed to represent the mechanism for effecting voluntary 



movements, and according to the 

 general schema the voluntary motor 

 path from cerebrum to muscle com- 

 prises two neurons, the pyramidal 

 neuron and the spinal or the cranial 

 neuron. Moreover, as represented 

 in the schema, the innervation is 

 crossed, the right side of the brain 

 controlling the musculature of the 

 left side of the body and vice versa. 

 As we shall see, however, when we 

 come to study the motor areas of 

 the brain, this rule has important 

 exceptions, and histologically there 

 is proof that some of the fibers in 

 each pyramid (2 in Fig. 74) con- 

 tinue into and terminate in the cord 

 on the same side. The pyramidal 

 system varies, in an interesting way, 

 in the extent of its development 

 among the different vertebrates. 

 It reaches its highest development 

 in man and the anthropoid apes. 

 In the other mammalia it is rela- 

 tively less important and the direct 

 tract in the anterior columns is 

 lacking altogether. In the birds 

 what represents the same system 

 is found in the anterior columns 

 (Sandmeyer), while in the frog the 

 system does not exist at all. 



The relative importance of the 

 system in the different mammalia 

 is indicated in the accompanying table taken from Lenhossek,* 

 in which the area of the pyramidal system is given in percentage 

 of the total cross-area of the cord: 



Mouse 1.14 per cent. 



Guinea pig 3.0 " 



Rabbit 5.3 " 



Cat 7.76 



Man.. ..11.87 



Fig. 74. Schema representing 

 the course of the fibers of the pyram- 

 idal tract : 1 , Fibers to the nuclei of 

 the cranial nerve; 2, uncrossed fibers 

 to the lateral pyramidal tract; 3, fi- 

 bers to the anterior pyramidal tract 

 crossing in the cord; 4 and 5, fibers 

 that cross in the pyramidal decussa- 

 tion to make the lateral pyramidal 

 tract of the opposite side. 



* Lenhossek, "Bau des Nervensystems," second edition, 1895. 



