432 



THE CENTRAL NERVOUS SYSTEM. 



When we take into consideration the simplest possible case, that 

 of the motor segment of such a neurone-chain, we find, for instance 

 (Fig. 348), that the neuraxis of a pyramidal cell in the brain cortex 

 (psychic cell) enters the white substance and traverses it as a nerve- 

 fiber through the peduncle and the pyramid into the crossed 

 pyramidal tract of the opposite side. Here its telodendria come in 

 contact with those of the motor neurone of the anterior horn. 



In the foregoing instance the motor nerve tract is composed of 

 two neurones of a motor neurone of the first order, extending 

 from the cortex of the brain to the anterior cornua of the spinal 

 cord, and of a motor neurone of the second order, the elements 

 of which extend from the anterior cornua to the telodendria in the 

 muscle. 



d 



Fig. 347. Schematic diagram of a sensorimotor reflex cycle ; sagittal section of 

 the spinal cord: C 1 , Motor cells of the anterior cornua; n, ti, neuraxes ; sN, sensory 

 neurone ; C 2 , spinal ganglion cell ; C, collaterals of the sensory neuraxes ; d, clendrite of 

 sensory neurone ; the broken lines at the cells on the left indicate their dendrites. 



The sensory tract may likewise be composed of neurones of the 

 first and second orders. The cellulifugal neuraxis arising from a 

 cell of the spinal ganglion passes to the posterior column of the 

 cord, gives off collaterals to the latter, and then passes upward by 

 means of its ascending branch through the posterior column to the 

 medulla. Although here the relationship is hot so clearly defined 

 as in the motor tract, it may nevertheless be assumed that the cellu- 

 lifugal (but centripetally conducting) neuraxis at some point or 

 other terminates in telodendria (sensory neurone of the first order), 

 which enter into contact with the corresponding structures of a cell 

 of the spinal cord or medulla oblongata. These cells would then 



