372 THE SOLIDS. 



vous cell. This secondary deposit is a fatty white-coloured 

 substance, and it is through this that the nerve acquires 

 its opacity. With the advance of the secondary deposit, 

 the fibrils become so thick, that the double outline of their 

 parietes comes into view, and^hey acquire a tubular appear- 

 ance. 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. The remaining cavity appears 

 to be filled by a pretty consistent substance, the band of 

 Remak, and discovered by him. In the adult, a nerve, con- 

 sequently, 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 ; 3d, of 

 a substance, which is frequently firm or consistent, included 

 within the cells, the band of Remak." 



The author's observations lead him to entertain views of 

 the development of the primitive nerve tube essentially dis- 

 tinct from those expressed by Schwann in the account quoted 

 above: according to these observations, the outer covering 

 of the nerve tube does not consist of a structureless membrane, 

 as is supposed by Schwann, as well as most other observers, 

 but is constituted of a nucleated form of fibrous tissue. (See 

 Plate XLIV. fig. 2.) Now, the nuclei observed by Schwann 

 the author believes to be concerned in the development of the 

 fibres, of which the proper membrane of each primitive nerve 

 tube is wholly constituted, and that they do not by their 

 growth give origin to a transparent tubular membrane. 



In the nerve tubes of both young and adult animals, 

 smooth bodies of an oval form and large size, their diameter 

 exceeding that of the tube itself, may always be observed ; 

 the nature of these, and the part taken by them in the struc- 

 ture and development of the nerve tube, I have not been able 

 to determine. 



Development of Glandular Nerve or Ganylionary Cells. The 



