THE NERVOUS SYSTEM. 379 



115. The cerebellum, with respect to the distribution of the ele- 

 mentary tissues, exhibits tolerably simple conditions, gray substance 

 occurring only on the surface of the convolutions, in the nucleus* den- 

 tatus, and in the roof of the fourth ventricle ; all the remainder con- 

 sists of white substance. The latter is wholly constituted of parallel, 

 probably unbranched, dark-bordered nerve-fibres, possessing all the 

 characters of central fibres (softness, proneness to become varicose, easy 

 isolation of the axis-cylinder, &c.), are essentially alike in all situations, 

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

 0-0012-0-004 of a line in the extremes, and of 0.002 of a line in the 

 mean. The gray 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 of a line, scattered 

 in the white substance, and recognizable by a sharp eye without further 

 aid (the substantia ferruginea superior) ; and, secondly, in the nucleus 

 dentatus, the grayish-red lamella of which contains a considerable 

 number of yellowish pigment nerve-cells of a medium, size (0-008- 

 0-016 of a line) 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 gray substance on the surface of the convolutions 

 of the cerebellum are more complex (vide " Mikr. Anat.," PL IV. fig. 4). 

 It consists everywhere, as is well known, of a layer, internally of a 

 rusty color, externally gray, which, except in the fissures, where the 

 internal layer is most usually thicker, presents pretty nearly the same, 

 but not everywhere an equal thickness. 



The internal ferruginous 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 gray layer, but are broken 

 up into numerous, for the most part, fine fasciculi, which are much inter- 

 laced, so that the whole ferrugineous layer is penetrated by a close but 

 delicate network of nerve-fibres, which recalls in appearance the ter- 

 minal plexuses in peripheral parts, as for instance, in the n. acusticus, 

 in the follicles of the vibrissw, &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 of a line, 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-fibres of 

 the white substance become gradually attenuated, most of them to a 



