392 SPECIAL HISTOLOGY. 



Several authors have noticed divisions of the nerve-tubes in the central 

 organs, such as, among the older ones, Ehrenberg, Volkmann, E. H. 

 Weber, and more lately also, Hessling ("Fror. N. Notiz.," Ap. 1849, 

 Jenaische, Ann. I. p. 283), E. Harless (ibid., p. 284), and SchafFner 

 (" Zeits. f. rat. Med.," IX.) in the brain of various vertebrate animals, 

 especially at the junction of the white and gray substance. I am not 

 inclined to doubt these statements, especially the latter, but I cannot 

 avoid the remark, that in the human brain, I have, hitherto, in vain 

 sought for divisions of this kind, and have had many hundreds of fibres 

 from the gray substance before me, under the most favorable circum- 

 stances, which presented no indications of the sort, whilst I have inva- 

 riably found such divisions in the spinal cord {vid. supra). The many- 

 rayed nerve-cells with branched processes are not as yet fully known in 

 all their relations. I have described their processes (as will be univer- 

 sally allowed, correctly), as a sort of pale, non-medullated nerve-tubes, 

 and have isolated them occasionally to the extent of | and J of a 

 line, without being able to notice anything more with regard to their 

 termination, than the fact of their ultimately assuming an extreme de- 

 gree of fineness. R. Wagner states, that those processes, which do not 

 pass into dark-bordered nerve-tubes, serve to connect the separate nerve- 

 cells together, but in so doing he manifestly says more than actual 

 observation warrants, as he has, hitherto, seen such a connection, only 

 in the electric lobes of the Ray. In the present state of neural Anatomy 

 there is nothing which should be more carefully avoided than the general 

 application of isolated observations, and I am therefore of opinion that 

 this question must as yet be regarded as an open one. It may indeed 

 be very consonant with physiological considerations, to explain the reflex 

 and alternating actions of separate sections of nerves by such connec- 

 tions between the cells, but it is precisely for that reason, that we should 

 be the more careful, and the more so because less obvious theories explain 

 the conditions just as well. I conclude, therefore, from the observations 

 hitherto made, only this much, that nerve-cells may anastomose, leaving 

 it to future inquiries to decide, whether they do so universally and with 

 all their processes, or whether in certain situations the latter do not 

 stretch out without any attachment, exerting a mutual influence and 

 affecting the nerve-fibres simply by juxtaposition, as appears to be the 

 case in the large nerve-cells of the cord and the roots of the spinal 

 nerves. 



§ 118. Membranes and Vessels of the central Nervous System. — 

 A. Membranes. 1. Spinal cord. The dura mater s. meninx fibrosa is a 

 whitish yellow, occasionally glistening, firm, tolerably elastic membrane, 

 consisting of parallel and mostly longitudinal fasciculi of connective tis- 

 sue, and of a fine, elastic, fibrous network in almost equal proportions. 



