STKUCTUKE OF THE NERVE-CENTRES. 



577 



been detached in preparing specimens for examination. Unipolar cells exist in some of 

 the lower orders of animals, but their presence in the human subject is doubtful. Bipo- 

 lar cells are found in the ganglia of the posterior roots of the spinal nerves, where they 

 are of considerable size. Smaller bipolar cells are found in the sympathetic ganglia. 

 Multipolar cells present three or more prolongations. 



Small cells, with three, and rarely four prolongations, are found in the posterior cor- 

 nua of the gray matter of the spinal cord. From their situation they have been called 

 sensory cells. They are undoubtedly found in greatest number in parts known to be 

 endowed exclusively with sensory properties. 



Large, irregularly-shaped multipolar cells, with numerous prolongations, are found 

 chiefly in the anterior cornua of the gray matter of the spinal cord, and these have been 

 called motor cells. They sometimes present as many as ten or twelve poles. 



With all these differences in the size and form of the nerve-cells, they present toler- 

 ably uniform general characters as regards their structure and contents. Leaving out the 

 apolar and unipolar cells, the perfectly-developed cells are of an exceedingly irregular 

 shape, with strongly-refracting, granular contents, frequently a considerable number of pig- 

 mentary granules, and with a distinct nucleus and nucleolus. The nucleus in the adult is 



FIG. 182. Nerve-cell from the ferruginous substance which forms the floor of the rhornboidal sinus, in man; 



magnified 350 diameters. (Kolliker.) 



almost invariably single, although, in very rare instances, two have been observed. Cells 

 with multiple nuclei are often observed in young animals. The nucleoli are usually single 

 but there may be as many as four or five. The strongly-refracting contents, the peculiar 

 shape, and the poles or prolongations, give to the nerve-cells an exceedingly characteristic 

 appearance, which is represented in Fig. 182. 



The diameter of the cells is as variable as their form. They usually measure from 

 T^Vo to TffT f an i ncn b ut there are many of larger size, and some are smaller. The 

 nuclei measure from ^-J^ to -j^Vo * an inch. 



The nerve-cells are so delicate and so prone to alteration, that their study is exceed- 

 37 



