THE NERVOUS SYSTEM. 1)1 



axis-cylinder, &c.), are essentially alike in all situations, as far 

 as their condition can be observed, and present a diameter of 

 0-0012 — 0-00-1'" in the extremes, and of 0-002'" in the mean. 

 The grey substance occurs, in the first place, very scantily in 

 the roof of the fourth ventricle above the velum medullare 

 inferius, in the form of brown nerve-cells, measuring 0*02 — 

 0-03'", scattered in the white substance, and recognisable by a 

 sharp eye without further aid, (the substantia ferruginea 

 superior) ; and, secondly, in the nucleus dentatus, the greyish 

 red lamella of which contains a considerable number of yel- 

 lowish pigment nerve-cells of a medium size (0-008 — 0-016'"), 

 with four or five processes, and which have no direct connection 

 with numerous nerve-fibres proceeding from the nucleus dentatus 

 into the medullary substance of the hemispheres, which pass 

 through among them. 



The relations of the grey substance on the surface of the 

 convolutions of the cerebellum are more complex, (vide 

 'Mikrosk. Anatomie/ PI. IV, fig. 4). It consists everywhere, 

 as is well known, of a layer, internally of a rusty colour, 

 externally grey, which, except in the fissures, where the 

 internal layer is most usually thicker, present pretty nearly the 

 same, but not everywhere an equal thickness. 



The internal ferrugineous layer contains nerve-fibres and large 

 masses of free nuclei. The former arise, without exception, 

 from the white substance, and run, in general, parallel to 

 each other, although on a transverse section of any convolution 

 slightly diverging in a penicillar manner, directly into the 

 ferrugineous layer. Within this layer they also run from 

 within to without as far as the grey layer, but are broken up 

 into numerous, for the most part, fine fasciculi, which are 

 much interlaced, so that the whole ferrugineous layer is pene- 

 trated by a close but delicate network of nerve-fibres, which 

 recalls in appearance the terminal plexuses in peripheral parts, 

 as, for instance, in the n. acusticus, in the follicles of the 

 vibrissa, &c. In the meshes formed by these nerve-fibres 

 lie a vast number of opaque, round, corpuscles, measuring 

 0-002 — 0-004'", in the mean 0-003'", which are nothing else 

 than free nuclei, and which frequently also exhibit a distinct 

 nucleolus, and not unfrequently other granules. 



In their passage through the ferrugineous layer, the nerve- 



