SPINAL CORD AS A PATH OF CONDUCTION. 179 



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 the cortex of the cerebral hemispheres in large pyramidal 

 cells; some of them cross the mid-line 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 com- 

 missure, terminate around the motor cells of the anterior columns 

 which give rise to the motor roots of the spinal nerves. Both 

 fasciculi, the lateral and the anterior, continue throughout the 

 length of the cord, diminishing in area on 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 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 mammalia 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: 



Mouse 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 

 varies in different animals, and it is necessary to bear this fact 



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

 fLenhossek, "Bau des Nervensystems," second edition, 1895. 



