662 



SIGNIFICANCE OF THE NERVOUS SYSTEM 



impulses from place to place within the realm of a single center. For this reason, 

 they generally remain confined to the central nervous system and serve chiefly 

 as intermediate conductors. This deduction seems the more correct, because their 

 axon usually splits into several branches within the gray matter, thus tending to 

 associate its different areas. The first and second types of Golgi cells are, of course, 

 multipolar in character. 



Type 3. — This cell is typically represented by the neurons forming the ganglia 

 upon the posterior root of the spinal cord and the ganglia occurring in the course 

 of the sensory branches of the cranial nerves. In lower forms (fish) and also 

 in the mammalian embryo, the cells of the spinal root ganglion possess two processes 

 which leave at opposite poles of the cell-body, and are, therefore, bipolar. In the 

 adult mammal, however, a union has been effected between them so that they now 

 arise as one (Fig. 278). The process passes away from the cell-body but soon di- 



FiG. 278. — Unipolar Cells of the Gasserlan Ganglion. 

 At a is shown the glomerulus formation of the axon. (CajcU.) 



vides into two, one of which extends into the posterior realm of the cord and the 

 other outward to the corresponding receptors. This peculiar distribution gives 

 rise to a unipolar cell with a T-shaped process, the branches of which become 

 meduUated and serve as long conducting fibers. It is questionable whether the 

 impulses conveyed inward from the distant receptor, must first of all enter the 

 cell-body proper before they can be transferred to the central branch. In fact, 

 one of the points regarding the fibrillar theory to be discussed later, is that the 

 cell-body is not necessary for conduction. It may be removed without disturbing 

 the passage of these afferent impulses, and hence, it must be concluded that the 

 dendrite-like distal branch is in direct functional relation with the axon-like 

 central branch. 



