NEEYOUS SYSTEM JLND GEXEEAL SENSATION. 



31 



where smaller branches are sent off transversely, however, 

 (fig, 10, b, b,) the structure of the primary fibres running in 

 a parallel direction 

 may be seen as dis- 

 tinctly as when they 

 are separated by 

 art. It frequently 

 happens that we may 

 tear fresh primitive 

 fibres in such a way 

 that the broader, 

 clear, middle por- 

 tion alone retains 

 its continuity, the 

 bounding lines hav- 

 ing given way trans- 

 versely ; the middle 

 portion is then seen 

 to be enclosed with- 

 in an extremely de- 

 licate contour. From 

 all this, it may be 

 inferred that each 

 primitive fibre con- 

 sists of A very clear 

 included substance, 

 and a transparent tubular sheath. The double line or contour 

 of either side being the optical expression of the inner and 

 outer wall of this tube. Other observers admit a more com- 

 pound structure, and some have even spoken of a ciliary epi- 

 thelium, lining the inner aspect of the sheath. 



[81. These primary tubes or fibres of the peripheral 

 nerves are similar, with very slight modifications, in every 

 part of the nervous system. It is necessary, however, to ex- 

 cept from this general rule the first and second cerebral 

 nerves. In the auditory nerve the fibres are somewhat more 

 delicate than elsewhere. They also very commonly appear 

 rather finer than wont where they traverse ganglions. They 

 appear to be distributed over the periphery of the body, with- 

 out, in any instance, anastomosing. They have a central and 

 a peripheral termination. With reference to the first, or 



Fi ^ 10. Branch of a nerve distributed to one 

 of the muscles of the e > re of a s P arrow - 



