NINETEENTH LECTURE. 



NERVE TISSUE. 



We turn to the final and highest histological formation of 

 the animal body : we refer to the nerve tissue. 



This has been included among the so-called " compound 

 tissues," that is those which possess more than one element. 

 And, in fact, we here meet with two such, namely, fibres and 

 cells. The former bear the name of the nerve fibres, nerve 

 tubes or primitive fibres ; the latter are called nerve cells or 

 ganglion bodies. 



The human nerve fibres appear either as dark contoured 



medullated elements (Fig. 167) or as 

 pale non-medullated ones (Fig. 172, b). 

 Since the former constitute by far 

 the most widely extended and impor- 

 tant peripheral elements, let us begin 

 our discussion with them. 



They are, like the non-medullated, 

 for long distances unramified fila- 

 ments, but of very unequal diameter, 

 from 0.0226 to 0.0018 mm. and less. 

 We distinguish, accordingly, broad or 

 coarse nerve fibres (Fig. 167, a) and 

 fine or narrow ones {c, d, c). Inter- 

 mediately between these appear the 

 ill \lj nerve tubes of medium width (b). 



V \ II I"' (\ Let us commence our investigations 



of the structure with the coarse, medul- 

 lated elements. 



Fresh and living, it appears like a 

 thread of a homogeneous milk-glass-like substance. We 

 recognize in it no further composition. 



a 



Fig. 167.— Human nerve 

 fibres : a, broad ; 6, medium 

 breadth ; c, d, e, fine. 



