16 ON TIIK NEUVOUS SYSTEM IV. 



ing to this view, is developed, not necessarily at the beginning or end of a tuhe, 

 but in its course.* I have not been, able to detect such a connection in any 

 observations which I have made on the brains of Frogs. The thin walls of the 

 cerebral lobes of these animals, especially when young, seemed to offer a favor- 

 able opportunity to test the correctness of this statement. If a piece, comprising 

 the whole thickness of the upper part of the lobes above mentioned, be placed 

 under the microscope, little else can be seen than the vesicular element, though the 

 section is sufficiently transparent to be viewed through its whole thickness. Here 

 and there a questionable tube may be found, but the principal structure, besides 

 the cells, is that of the capillary vessels, which may be identified by their containing 

 blood discs, and may be frequently traced dwindling into minute empty tubes, 

 which are easily mistaken for nerve tubes. If a piece of a lobe which has been 

 macerated in alcohol be transferred to turpentine, the preparation soon becomes 

 transparent, when its minute structure can be determined. In the cerebral lobes 

 of Frogs thus treated, spherical cells and capillary vessels are easily detected, but 

 no nerve tubes. The longest portion of the whole mass obviously consists of cells, 

 and, after repeated examinations both of adults and embryos, not one has been 

 found presenting the caudate condition. Here, then, at least, we have an instance 

 in which the cells and nerve tubes do not come together, in which they are not 

 immediately connected ; and, as will be seen in the spinal chord, continuity is not 

 necessary for the manifestation of nervous force, nor is contact even. In other parts, 

 as the cerebellum, optic lobes, optic thalami, the tubes and cells are more or less 

 intricately combined, the first forming an interlacement, in the meshes of which the 

 cells are inclosed ; but even here no continuity between the tw r o was seen, not a 

 single instance in which a nerve tube arose from or terminated in a cell. The cells 

 (Fig. 10, Plate I.) are quite uniform in appearance, from whatever part of the 

 brain they may be derived. The only variation is that of size. The cell wall is 

 quite thin, and so easily ruptured, that it sustains but little pressure, and, unless 

 great care be taken to protect it, it will be found quite difficult to obtain a fair 

 view of even a single one. Within, the cell contains numerous minute granules, 

 which fill the larger portion of its cavity, leaving a thin transparent space around 

 the circumference. The nerve tubes, whether from the base of the brain or from the 

 chord, are varicose in nearly every instance ; and whether this condition be accidental 

 or not, when manipulated with the greatest care, whether viewed with or without 

 pressure, even when perfectly fresh and almost living, they were never seen in any 

 other condition. Every attempt to determine the mode of termination of the nerve 

 tube was unsuccessful. If, however, the views of Wagner and some of the more 

 recent writers be correct, namely, that, before the nerves terminate, the " white 

 substance of Schwann " disappears, so that the tube contracts and becomes invisible, 

 except with the highest powers of the microscope, we have in part an explanation 

 of the difficulty. Wagner maintains that these minute tubes are lost on the walls 



* Zur Lehre von dem Verhaltniss der Ganglien-korper zu den Nerven-fasern. Neue Beitrage von 

 Dr. F. H. Bidder. Nebst einem Anhange von Dr. A. W. Volkman. Leipzig. 1847. 



