228 INNERVATION. [CHAP.VIII. 



in rendering the fibril dark is obvious. With the advance of the 

 secondary deposit, the fibrils become so thick, that the double 

 outline of their parietes comes into view, and they acquire a tubu- 

 lar appearance. On the occurrence of this secondary deposit the 

 nuclei of the cells are generally absorbed ; yet a few may still be 

 found to remain for some time longer, when they are observed lying 

 outwardly between the deposited substance and the cell-membrane, 

 as in the muscles (e). The remaining cavity appears to be filled 

 by a pretty consistent substance, the band of Remak, and dis- 

 covered by him. In the adult a nerve, consequently, consists, 

 1st, of an outer pale thin cell-membrane, the membrane of the 

 original constituent cells, which becomes visible, when the white 

 substance is destroyed by degrees ; 2nd, of a white fatty substance 

 deposited on the inner aspect of the cell-membrane, and of greater 

 or less thickness ; 3rd, of a substance, which is frequently firm or 

 consistent, included within the cells, the band of Remak." 



The fully-formed vesicular matter exhibits the persistent state of 

 the cells of primitive development. According to Schwann, the only 

 change which the full-grown cell exhibits consists in an increase of 

 size, and in the development of the pigmentary granules within. 

 According to Valentin's description, the following is the process of 

 development of the nerve- vesicles. In the very young embryos of 

 mammalia, as the sheep or calf, the cerebral mass in the course of 

 formation contains, in the midst of a liquid and transparent blas- 

 tema, transparent cells of great delicacy with a reddish-yellow 

 nucleus. Around these primitive cells, which we find likewise 

 formed after the same type in the spinal cord, a finely granular 

 mass becomes deposited, which probably is not at first surrounded 

 by an enveloping cell-membrane. At this early period of formation 

 the primitive cell still preserves its first delicacy to such a degree, 

 that the action of water causes it to burst immediately. In propor- 

 tion as the granular mass contracts itself within certain limits, a cell- 

 membrane probably is developed around it, so that the vesicle gra- 

 dually acquires the exact form and size, and its contents the proper 

 characters, which belong to the fully -formed nervous corpuscle. 



Of the Regeneration of Nervous Matter. Our chief knowledge 

 on this subject is with respect to the regeneration of the tubular 

 fibres. Many years ago our countryman, Dr. Haighton. in making 

 experiments to determine the functions of the vagus nerve, shewed 

 that when a nerve is simply divided, without removing any portion 

 of it, union would take place, and the nerve resume its proper 

 office. If a considerable piece were excised, so as to leave much 



