NERVES. 359 



may frequently be observed, forming a kind of capsule 

 around them, which, however, is entirely external to the 

 proper membrane of the cells. (See Plate XLV. Jig. 4.) 



Tubular Structure. The white fibrous substance of the 

 brain, spinal cord, the nerves of motion and of special sen- 

 sation, is composed of unbranched tubules, the diameter 

 of which is subject to considerable variation. The tubules 

 of the cerebrum are exceedingly slender, as are also those 

 of the nerves of special sense : those of the cerebellum, 

 spinal cord, posterior root of spinal nerves, and of the sym- 

 pathetic system are of somewhat larger calibre, while those 

 of the motor nerves are of still larger size, and of firmer 

 texture. See the figures. 



The tubules of the white substance of the cerebrum are 

 especially prone to become dilated at intervals, or varicose. 

 (See Plate XLIV.j^. 7.) This condition was formerly sup- 

 posed to be natural, and it was presumed, that by this cha- 

 racter the nerves of special sense could be discriminated from 

 those of motion : there is little doubt, however, but that this 

 varicose condition of the fibres is abnormal, and that it is pro- 

 duced by the pressure and disturbance to which they are sub- 

 ject during examination. The tubules of the cerebellum are 

 subject to a like change, although in a less degree ; those 

 of the nerves of motion are but little prone to the altera- 

 tion, these becoming, when much disturbed, broken into frag- 

 ments, many of which assume a globular form, and all of 

 which are greatly corrugated. (See Plate XLIV. Jig. 1.) 



The nerve tubules contain a fluid matter, and it is the 

 collection of this fluid in certain parts of each tubule, the 

 result of pressure, which occasions the distension of the 

 membranous wall of the tubes, and which gives rise to the 

 varicose condition described. Such, is at least, the most 

 probable explanation of the exact nature of this condition. 



The tubules of the cerebrum, cerebellum, spinal cord, 

 and motor nerves, &c., present an average diameter : never- 

 theless, much difference may be detected in the size of the 

 tubules taken from the same portion of the nervous system : 



F F 



