358 SPECIAL HISTOLOGY. 



rent contents, and one, or more rarely several, large opaque nucleoli, 

 which occasionally exhibit a cavity. 



The size of the nerve-cells varies very much; like the fibres, they 

 occur as large, small, and middle-sized. The extreme dimensions of the 

 cells are 0-002-0-003, and 0-05-0-06 of a line. The nuclei, which for 

 the most part are in proportion to the cells, measure from 0-0015-0-008 

 of a line; the nucleoli 0*0005-0-003 of a line. The nerve-cells, more- 

 over, are distinguished according as they are : 1, thin or thick-walled, 

 of which the former are found almost wholly in the spinal cord and 

 brain ; and 2, as they are independent cells, or are furnished with pale 

 processes, of which they may have one, two, or several (uni-, bi-, multi- 

 polar cells), and which are frequently ramified, and the former, in many 

 situations, continuous with dark-bordered nerve-fibres, and even having 

 the nature of non-medullated nerve-fibres. Besides the nerve-cells, 

 there also exist in the gray-substance of the higher central organs, as 

 constant constituents, a finely granular pale substance, which has the 

 greatest resemblance to the contents of the cells, and besides this, in 

 places, large accumulations of free cell-nuclei. Similar elements are 

 contained in the retina, and according to Wagner and Robin in the 

 ganglia of the Plagiostomata. 



The nerve-cells are simple cells, as which they were understood even 

 by Schwann ; this is clearly and manifestly shown by their form, their 

 chemical composition, and their development. When Bidder, more 

 lately (1. c.), relying upon the fact that the nerve-cells in many situa- 

 tions are in connection at each end with dark-bordered nerve-fibres, pro- 

 pounds the opinion that they are membraneless masses, imbedded in 

 dilatations of the nerve-tubes, he has overlooked those cells from which 

 no fibres are given off, which possess exactly the same membrane as 

 those with processes ; and has not considered that there also exist nerve- 

 cells with a single, and others with numerous processes, as applied to 

 which, his view would be altogether unnatural ; and lastly, that the 

 development of these bodies indicates that the nerve-cell is formed in 

 toto, whether it possess processes or not, from a simple cell. It has not 

 yet been determined whether the nerve-cells of the large central organs 

 have membranes or not ; Stannius was unable to detect them in the 

 Lamprey, and R. Wagner says the same of the nerve-corpuscles of the 

 electric lobes of the Ray. I think I have seen a membrane in the large, 

 many-rayed corpuscles in the spinal-cord and cerebellum of man, and 

 occasionally, also, in others, but I freely acknowledge that no membrane 

 can be detected in all the smaller cells, nor in the processes of the cen- 

 tral cells in general. This does not, however, appear sufficient to justify 

 the denial of the existence of membranes in these instances, and I 

 believe, that in this case, as in that of the finest nerve-tubes, we must 



