380 



BIOLOGY. 



[Book vi. 



It is only by the presence of this cellular expansion at a point 

 of their course that the sensitive fibres are distinguished from 

 the motory fibres. In a general manner they seem also to be 

 less voluminous ; but the diameter of the nervous fibres of every 

 kind is a very variable thing. 



A third variety of fibres exists especially in the cords of the 

 great sympathetic nerve. These are the grey, gdatmiform, or 



.^•-'l 



e 



S 



-I 



Fig. 69. 



Section magnified and schematic of the spinal maiTow of man at the commencement of 

 the lumbar region ; a, posterior iissure ; &, anterior fissure ; c, central canal; d, posterior 

 cords ; c, lateral cords ; /, anterior cords ; g, anterior commissure of the white sub- 

 stance ; h, posterior horns ; i, anterior horns ; fc, posterior commissure ; I, anterior com- 

 missure of the grey substance; m, gelatinous substance ; 7t, posterior roots ; and o, anterior 

 roots of the nerves ; 2>, ganglion of the posterior root ; q, mixed nerve from the union of 

 the two roots. 



Remah's fibres. They constitute almost the totality of all the 

 cords or grey threads of the great sympathetic, and are on the 

 other hand rare or absent in the white branches of the same 

 system. These fibres are thin (0'™,003), pale, greyish, be- 

 strewn with fine granulations of the same colom-, and even 

 with elliptical nuclei (Fig. 70). They dominate in the threads 

 of the great sympathetic, and are lacking in the others. As 

 M. Ch. Robin remarks, they seem to be foetal nervous fibres. 

 In efiect all the white fibres of the animal life assume in the 

 first phase of their histological evolution the form of gelatini- 



