518 



NERVOUS SYSTEM. 



FIG. 183. Unipolar nerve- 

 cell with a spiral fibre 

 (Landois). 



FIG. 184. Bipolar 

 nerve-cell (Landois). 



which is continuous with a nerve-fibre. These cells frequently have a con- 

 nective-tissue envelope, or sheath, which is prolonged as a sheath for the 



nerve. Unipolar cells, with a connect- 

 ive-tissue sheath, the pole being sur- 

 rounded by 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 medul- 

 lated nerve-fibres. Usually the medul- 

 lary substance does not extend over 

 the cell, although this sometimes oc- 

 curs. 



Multipolar cells have a number of 

 poles, but there is always one pole 

 which does not branch and which becomes continuous with the axis-cylinder 

 of a nerve-fibre. This is called the axis-cylinder prolongation. Of the 

 other poles, some are continuous with poles of contiguous cells, connecting 

 numbers of cells into groups, and others, 

 which are sometimes called protoplasmic 

 prolongations, branch freely and are lost 

 in the intercellular substance. 



With all the differences in the size 

 and form of the nerve-cells, they present 

 tolerably uniform general characters as 

 regards their structure and contents. 

 With the exception of the unipolar and 

 bipolar cells, they are irregular in shape, 

 with strongly refracting, granular con- 

 tents, frequently a considerable number 

 of pigmentary granules, and always a dis- 

 tinct nucleus and nucleolus. The nucleus 

 in the adult is almost invariably single, 

 although, in rare instances, two have been FlG 



Observed. Cells with multiple nuclei are z, axis-cylinder prolongation ; Y, protoplas- 

 ... , . i mi mic branches. 



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 ^^ of an 

 inch (20 to 50 /w,) ; but there are many of larger size and some are smaller. 

 The nuclei measure ^-gW to y^Vo- of an inch (12 to 20 /*). The nerve-cells 

 are soft, have no true cell-membrane and are fibrillated, the fibrillation ex- 

 tending to the poles. The transverse striae in the axis-cylinder treated with 

 silver nitrate, noted by Fromann and confirmed by Grandry and others, 



