53 



TEXT-BOOK OF PHYSIOLOGY 



In the posterior horn nerve-cells are also present, though they are not 

 so numerous as in the anterior horn. At the base of the horn and on its 

 inner side there is a well-marked group of cells which extends from the 

 seventh or eighth cervical nerves downward to the second or third lumbar 

 nerves, being most prominent in the thoracic region. This column is known 

 as Clarke's vesicular column. From the nerve-cells constituting this column 

 axons pass obliquely outward into the portion of the white matter known 

 as the direct cerebellar tract. Other nerve-cells send their axons into the 

 white matter in the posterior portion of the cord bordering the posterior 

 median fissure. Some of the nerve-cells, their situation and the distribu- 

 tion of their axons are shown in Fig. 228. 



Donai 



l/entral 



FIG. 228. SCHEME OF THE STRUCTURE OF THE CORD. (Howell after Lenhossek.) On the 

 right the nerve-cells; on the left the entering nerve-fibers. Right side: i, Motor cells, anterior 

 horn, giving rise to the fibers of the anterior root; 2, tract cells whose axons pass into the white 

 matter of the anterior and lateral columns; 3, commissural cells whose axons pass chiefly through 

 the anterior commissure to reach the anterior columns of the other side; 4, Golgi cells (second type), 

 whose axons do not leave the gray matter; 5, tract cells whose axons pass into the white matter 

 of the posterior column. Left side: i, Entering fibers of the posterior root, ending, from within 

 outward, as follows: Clarke's column, posterior horn of opposite side, anterior horn same side (re- 

 flex arc), lateral horn of same side, posterior horn of same side; 2, collaterals from fibers in the 

 anterior and lateral columns; 3, collaterals of descending pyramidal fibers ending around motor 

 cells in anterior horn. 



Classification of Nerve-cells. The cells of the gray matter may be 

 divided into three main groups: viz., intrinsic or associative, receptive or 

 afferent, and emissive or efferent. 



The intrinsic cells are associative in function. The axons to which these 

 cells give origin pass more or less horizontally into the white matter, where 

 they divide into two branches, one of which passes upward, the other down- 

 ward. At various levels they reenter the gray matter and arborize around 

 other intrinsic cells. 



The receptive cells are largely sentient or afferent in function, inasmuch 

 as the excitations brought to the spinal cord by the afferent nerves in the 

 dorsal roots from the general periphery are received by them and transmitted 

 by and through their axons to the cortex of the cerebrum, where they are 

 translated into conscious sensations. As the nerve-fibers in the dorsal roots 

 of the spinal nerves are classified, in accordance with the sensations to which 



