46 NERVOUS SYSTEM. 



enveloping some of the cells, and vessels. The most inter- 

 esting and important of these structures, in their physiologi- 

 cal relations, are the cells and the prolongations by which 

 they are connected with the nerves. 



Nerve-cells. Anatomists are now pretty well agreed that 

 the following varieties of cells exist in the nerve-centres, and 

 constitute their essential anatomical elements ; viz., apolar, 

 unipolar, bipolar, and multipolar cells. Although some have 

 denied the existence of apolar cells, there can be little doubt 

 of their presence in the centres in small numbers, and, as is 

 suggested by Kolliker, they may be nerve-cells in an imper- 

 fect state of development. The nerve-cells present great 

 differences in their size and general appearance, and some 

 distinct varieties are found in particular portions of the 

 nervous system, and are probably connected with special 

 functions. 



The apolar cells are simply rounded bodies, with granular 

 contents, a nucleus and nucleolus like other cells, but with- 

 out any prolongations connecting them with the nerve-fibres. 

 They have been observed in the cerebro-spinal centres, and 

 they always exist in the sympathetic ganglia. Those who 

 deny their existence believe that the poles have been de- 

 tached 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. Bipolar 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 cornua of the gray matter of the 

 spinal cord. From their situation they have been called 

 sensitive cells. They are undoubtedly found in greatest 

 number in parts known to be endowed exclusively with 

 sensitive properties. 



