430 SPECIAL HISTOLOGY. 



Tadpole, did not similar conditions most probably also obtain in many 

 other terminations of nerves. This is certain as regards those of the 

 electric organ of the Ray, which, even when developed, agree in many 

 respects with those of the more advanced Tadpole, and as Ecker has 

 lately shown (" Zeitsch. fur wissensch. Zoologie," 1849, p. 38), are de- 

 veloped in precisely the same manner. The nerves, also, in the skin of 

 the Mouse {vid. note to § 121), evidently belong to the same category ; 

 and consequently it may hereafter be shown, that wherever peripheral 

 divisions of nerves occur, their development proceeds essentially as it is 

 here described. 



With respect to the development of the nerve-filres in the central 

 organs, we possess but few researches. Of the fibres in the ganglia, I 

 can only observe, that they are developed subsequently to those of the 

 nerves, and probably from smaller fusiform cells, which may be noticed 

 in association with the nerve-cells. On one occasion, in a spinal gan- 

 glion of a four months' human embryo, I noticed a cell of this kind in 

 connection with the process of a nerve-cell (Fig. 164). The formation of 

 the fibres in the cord and brain is extremely difficult of investigation, 

 and is best studied with the aid of chromic acid. In the human embryo, 

 I find, as early as the end of the second month, the commencement of 

 the formation of the tubules in question, the white substance being dis- 

 tinctly finely striated, and manifestly containing, in places, very delicate 

 fusiform cells, which are sometimes independent or isolated, sometimes 

 connected, two or three, or several together (Fig. 164). All these cells 

 are at first pale, investing the nucleus, which measures 0*002-0*003 of a 

 line quite closely, and having processes almost as fine as the fibrils of 

 connective tissue. In the fourth month, when the two kinds of sub- 

 stance are quite distinct, nuclei are still occasionally to be seen in the 

 now wider fibres, but in some they have disappeared, although the fibres 

 are without dark contours ; which are not developed before the middle 

 period of foetal life (in the foetal Calf, when more than 12 inches long, 

 according to Valentin), and, indeed, first in the spinal cord. 



As regards the subsequent changes in the nerve-fibres, it has already 

 been remarked, that they occasionally increase very considerably in 

 thickness. According to Harting (1. c, p. 75), the fibres of the median 

 nerve which have not yet acquired dark contours, measure in a four 

 months' human embryo, on the average 3*4'"'", in a new-born child 

 10"4™'", in the adult 166"'™. The increased thickness of the nerves 

 themselves appears, according to Harting, from the fourth month 

 onwards, to depend solely upon the enlargement of the already existing 

 elements, the foetus and new-born child already possessing the same 

 number of primitive-fibres as the adult. 



It remains to be observed, that extremely hxf patJiological changes of 



