NERVE TISSUE. 



193 



The nerve tube is, however, a marvelously changeable 

 thing. Under our eyes, and against the will of the observer, 

 it changes its original appearance most rapidly into a second, 

 third cadaveric image. 



It is at present established that every broad nerve tube con- 

 sists of three elements. 



It is invested by a, as a rule, very fine homogeneous con- 

 nective-tissue envelope, the neurilemma, the Schwann's or 

 primitive sheath (Fig. 169, b, 171, e). The latter contains, from 

 point to point, an elongated nucleus. Occasionally the neu- 

 rilemma appears considerably thickened (Fig. 171, c). 



In the axis, occupying a fifth to a fourth of the entire 

 diameter, we recognize a pale cylindrical filament, formed of 



Fig. 168. — Human nerve 

 fibres in various stages of 

 coagulation. 



Fig. 160. — Various nerve fibres ; 

 a, after treatment with absolute 

 alcohol : £, with collodion ; c, 

 fibres of the lamprey ; d, from the 

 olfactory nerve of the calf; e and 

 y", from the human brain. 



an albuminous substance. This is the axis cylinder, the sole 

 essential portion of the nerve tube (Fig. 169, a, b, c, e, 171, e). 

 It is surrounded by the so-called nerve medulla or medullary 

 sheath, a peculiar and very delicate combination of albumin- 

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