178 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



midal fasciculus. Eventually, however, these latter fibers also 

 cross the mid-line in the anterior white commissure, not, however, 

 all at once, as at the pyramidal decussation, but some at the level 

 of each spinal nerve. These pyramidal fibers have their origin 

 in large pyramidal cells (Betz cells) lying in the cortex of the ante- 

 rior central convolution of the cerebrum; some of them cross 

 the mid-hne before reaching the medulla to end around the cells 

 of origin of the cranial nerves, but the greater number continue 

 into the cord and, after crossing the mid-line in the pyramidal 

 decussation or in the anterior white commissure, terminate 

 around the motor cells of the anterior columns which give rise 

 to the motor roots of the spinal nerves. Both fascicuH, the 

 lateral and the anterior, continue throughout the length of the 

 cord, diminishing in area on the way as some of theii- fibers ter- 

 minate 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 comprises two neurons, — the pyra- 

 midal or cerebrospinal neuron and the spinal or the cranial 

 neuron. Moreover, as represented in the schema, the innerva- 

 tion is crossed, the right side of the brain controlling the mus- 

 culature 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. 80). continue into and terminate in the cord on the same 

 side. The pyramidal system varies, among the different verte- 

 brates, in an interesting way, in the extent of its development and 

 in its location in the cord.* It reaches its highest development in 

 man and the anthropoid apes. In the other mammaha it is rela- 

 tively less important and the anterior fasciculus may be lacking 

 altogether. In the birds what represents the same system is 

 found in the anterior funiculus (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.f 

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

 of the total cross-area of the cord: 



Movise 1.14 per cent. 



Guinea pig 3.0 " 



Rabbit 5.3 



Cat 7.76 



Man 11.87 



Evidently, therefore, the importance of the pyramidal system 



* Simpson, "Quarterly Journal of Exp. Physiology," 8, 79, 1914. 

 t Lenhossek, "Bau des Nervensy stems," second edition, 1895. 



