88 



THE NERVOUS SYSTEM 



this substance its gray appearance. The ramification of dendrites and unmy- 

 elinated fibers forms a very intricate feltwork throughout the gray substance 

 (Fig: 64). 



The nerve-cells of the spinal cord vary greatly in size. The largest are 

 situated in the anterior column and may measure more than 100 micra. They 

 are all multipolar, possess each a single axon, and may be classified in four groups: 

 (1) Some of the cells, found in the posterior horn and particularly in the sub- 

 stantia gelatinosa Rolandi, belong to Golgi's Type II, with short axons confined 

 to the gray substance. These, however, are present in relatively small numbers 

 in the spinal cord. (2) The motor cells, situated in the anterior column and 



Fig. 64. From a section through the spinal cord of a monkey; showing part of the an- 

 terior gray column including a multipolar nerve-cell and the surrounding neuropil. Pyridin- 

 silver method. 



most numerous in the cervical and lumbar enlargements, are of large size and 

 possess axons which leave the cord in the ventral roots. (3) Smaller cells are 

 present in the lateral column in the thoracic region and give rise to the visceral 

 efferent fibers of the ventral roots (Fig. 37). (4) Other cells of small or medium 

 size, found chiefly in the posterior column, possess axons which pass into the 

 white matter, where they bend sharply to become ascending or descending 

 fibers, or divide dichotomously into ascending and descending branches (Fig. 

 68). Some of the ascending fibers reach the brain; the others merely connect 

 the different levels of the spinal cord. The fibers of the latter group constitute 

 the fasciculi proprii and vary greatly in length, some connecting adjacent, 



