196 



NINETEENTH LECTURE. 



constituents and the giving off of the products of decomposi- 

 tion. 



Let us now pass to the pale non-medullated nerve fibres. 

 Originally, in the foetal period, all the primitive tubes of 

 the entire nervous system were thus constituted. If we take 

 one of the lowest fishes, the lamprey (petromyzon), we meet 

 with this condition throughout its entire life (Fig. 169, c). A 

 nucleated sheath invests the axis cylinder. Medullated nerve 

 fibres are here entirely wanting. 



Let us turn, at a bound, to the highest animal being, to 

 man. 



In us, the olfactory nerve, alone, consists throughout of 

 pale, non-medullated fibres, as does in great 

 part the sympathetic with its ramifications. 

 These pale structures have been called 

 Remak's fibres. They appear as delicate 

 0.0038 to 0.0068 mm. wide, nucleated fila- 

 ments (Fig. 172, b). 



Does what has been mentioned above, 

 however, contain the entire structure of the 

 nerve fibre ? We now encounter this diffi- 

 cult question. 



It does not appear so ; nevertheless, we 

 are once more at the limits of the microscopy 

 of the present day. 



The axis cylinder, the best portion of the 

 nerve tube, most probably consists of a 

 bundle of extremely fine filaments. 



They (Fig. 173) appear to be embedded in 

 a delicate granular substance. They have 

 been called axis fibrillae (Waldeyer) or primitive fibrillae 

 (Schultze). Here, also, the incitation was furnished by a bril- 

 liant investigator, who has by no means been honored by his 

 cotemporaries in proportion to his merits, Remak, the 

 founder of the modern history of development. Many years 

 ago he saw this combination in the nerve fibres of the river- 

 crab. 



Fig. 172. — A sympa- 

 thetic nerve branch of 

 a mammalial animal ; 

 two dark bordered 

 nerve fibres, a, with an 

 excess of the Remak's 

 formation, b. 



