256 



NORMAL HISTOLOGY. 



trifugal impulses emanating from the motor cells in that region 

 (Fig. 238, e, and Fig. 236, j). The further transmission of these 

 centripetal stimuli toward the higher nerve-centres of the brain 

 probably takes place : first, through the cells in the posterior horns, 

 the neurites from which pass into the lateral columns and there 

 ascend the cord ; second, through the cells of Clarke's column, 

 which also send neurites into the lateral column, where they enter 

 the direct cerebellar tract (Fig. 236, o ; see also Fig. 239). In 

 addition to these centripetal or sensory neurites, the posterior nerve- 

 roots contain a few centrifugal neurites. 



Fig. 240. 



Diagram of a sensory and a motor tract. (R. yCajal.) A, psycho-motor region in cerebral 

 cortex ; B, spinal cord ; C, voluntary muscle ; D, spinal ganglion ; D', skin ; a, axis-cylin- 

 der of a neuron extending from the cerebral cortex to the anterior horn of the spinal 

 cord, where the terminal teleneurites are in relations with the teledendrites of the motor 

 cell at b. The sensory stimulus arising in the skin, U , is transmitted by the neuron 

 dDce to /, where it is communicated to the neuron fy. The point / may be in the cord 

 or in the medulla oblongata. 



In order to understand the origin of the anterior spinal nerve- 

 roots we must first consider the course of the centrifugal neurites in 

 the pyramidal tracts (Figs. 233, 234, 235). These enter the gray 

 matter and end in teleneurites, which are associated with the tele- 



