NERVE-CELLS 



469 



and these have- been called motor cells. They sometimes present as 

 many as ten or twelve poles. 



Unipolar cells, such as exist in the ganglia of the nerves as dis- 

 tinguished from the ganglia of the cerebro-spinal axis, have but a single 

 prolongation, which is continuous with a nerve-fibre. These cells 

 frequently have a connective-tissue envelope, or sheath, which is pro- 

 longed as a sheath for the nerve. Unipolar cells, with a connective- 

 tissue sheath, the pole surrounded with a spiral fibre, have been observed 

 in the sympathetic ganglia of the frog. These do not exist in the human 

 subject or in the mammalia, and nothing is known of the uses of the 

 spiral fibres. 



Bipolar cells seem to be nucleated enlargements in the course of 

 medullated nerve-fibres. Usually the medullary substance does not 

 extend over the cell, although this some- A B 



times occurs. 



Multipolar cells have a number of 

 poles, but there is always one pole which 

 does not branch and which becomes con- 

 tinuous with the axis-cylinder of a nerve- 

 fibre. This is now known as the neurite, 

 or axis-cylinder prolongation. The other 

 poles, called dendrites, or protoplasmic 

 prolongations, branch freely and are lost 

 in the intercellular substance (see Plate 

 XI, Fig. i). 



With all the differences in the size and 

 form of the nerve-cells, they present tol- 

 erably Uniform general Characters as re- a spir'al fibre; B', bipolar nerve-cell (Lan- 



gards their structure and contents. With 



the exception of the unipolar and bipolar cells, they are irregular in 

 shape, with strongly-refracting granular contents, frequently a consider- 

 able number of pigmentary granules, and always a distinct nucleus and 

 nucleolus. Many cells also contain little angular bodies called Nissl's 

 granules (see Plate XI, Fig. 2). The nucleus in the adult is almost 

 invariably single, although in rare instances two have been noted. Cells 

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

 usually are single, but there may be as many as four or five. The 

 diameter of the cells is variable. They usually measure y^Vo to -5-^ 

 of an inch (20 to 50 JJL) ; but there are many of larger size and some 

 are smaller. The nuclei measure 2~gVo to TaVo f an mc ^ ( I2 to 2O ^)- 

 The nerve-cells are soft, have no true cell-membrane and are fibrillated 

 the fibrillation extending into the poles. 



Fig. 108. A, unipolar nerve-cell with 



