10 



HISTOLOGY. 



faces minute vibratile hair-like processes (cilia), whence this 

 type is called ciliated epithelium. Again, epithelia may be 

 grouped according to function, and then cuticular, sensory, and 

 glandular epithelia may be recognized. Epithelia may give rise 

 to important structures, such as hair, feathers, scales, enamel of 

 the teeth, etc., which will be mentioned in the proper places. 



Nervous tissue arises from the ectoderm, and hence from 

 epithelium. It has for its purpose the recognition and trans- 

 ferrence of impulses, the perception of sensations, and the pro- 

 duction of other impulses which shall affect nervous or other 

 tissues. It has for its essential constituent nerve cells, to which 

 are usually added other cells of a supportive nature. Nerve 

 cells (or ganglion cells) consist of a central nucleated body 

 from which radiate one or more protoplasmic processes which, 



FIG. 9. Different forms of nerve cells from cat. A, pyramid cell 

 from cerebrum ; B, cell from spinal cord ; D, unipolar cell from spinal gan- 

 glion ; C, glia cell from spinal cord ; a, axis cylinder. 



after a longer or shorter course, break up into minute branches 

 or fibrillations. It must, however, be kept in mind that these 

 processes are really parts of the cell, although the term cell is 

 frequently restricted to the central mass, while the processes 

 are called nerve fibres, etc. When a nerve cell has two proto- 

 plasmic processes it is spoken of as bipolar ; when more than 

 two as multipolar. 1 One of these processes is of considerable 



i In some cases ' unipolar ' nerve cells are found ; but the process in these is soon 

 found to divide, its halves going off at right angles to the previous course, thus showing 

 that these cells are really bipolar. 



