SPINAL CORD AS A PATH OF CONDUCTION. 



169 



the pyramidal system. At the junction of the medulla and cord 

 the fibers of the pyramids decussate in part, forming a conspicuous 

 feature of the internal structure at this point known as the pyram- 

 idal decussation. According to the general schema of this decus- 

 sation (see Fig. 74), the larger number of the fibers in the pyramid 

 of one side pass over to form the 

 crossed pyramidal tract of the other 

 side of the cord (4, 5), while a 

 smaller part (3) continues down 

 on the same side to form the direct 

 pyramidal tract. Eventually, how- 

 ever, these latter fibers also cross 

 the mid-line in the anterior white 

 commissure, not, however, all at 

 once, as at the pyramidal decussa- 

 tion, 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 decussa- 

 tion or in the anterior white com- 

 missure terminate around the motor 

 cells of the anterior horns, which 

 give rise to the motor roots of the 

 spinal nerves. Both tracts, the 

 crossed and the so-called direct, 

 continue throughout the length of 

 the cord, diminishing in area on the 

 way as some of their fibers termin- 

 ate in each segment. This system 

 of fibers is supposed to represent 

 the mechanism for effecting volun- 

 tary movements, and according to the general schema the volun- 

 tary motor path from cerebrum to muscle comprises 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 im- 



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. 



