2^6 NERVOUS SYSTEM. [sect. Il6. 



contain, at first, tubes of o - ooi2'" to 0003"' in diameter, nearly- 

 all of which finally diminish to o'ooi'", and, in the grey substance, 

 assume the smallest diameter of the nerve-tubes, viz., 0"ooo4'" to 

 o - ooo8"'. The fibres given off from these bundles within the 

 greyish-red layer are, in part, of the same diameter as in the 

 bundles, as is especially the case with those of the thicker white 

 streak, and, in part, finer. The fibres passing from the bundles 

 into the superficial Avhite substance are also, as a rule, thicker, up 

 to o"003 / "; still there exist in this layer other very fine fibrils, of 

 o - oo04"'. Although the connection of the fibres entering into the 

 cortical substance with the nerve-cells has not, as yet, been actually 

 demonstrated, still I do not hesitate in affirming it ; and I regard 

 the cortical substance as the place of origin of all the nerve-fibres 

 of the hemispheres and corpus callosum. 



The corpus callosum is composed purely of medullary matter, 

 with parallel nerve-fibres of quite the same appearance and dia- 

 meter as those of the medullary substance of the hemispheres. 

 This is also the case with the commissura anterior and the fornix, 

 which, however, comes in contact with the grey substance in 

 various ways. The cornu Ammonis and the hippocampus minor 

 present almost the same relations as the convolutions of the 

 hemispheres. 



Lastly, we have still to speak of the origin of the first two pairs 

 of nerves. In the white part of the tractus olfactorius, the olfactory 

 nerve contains fine nerve-fibres of from o - ooo4'", to at most croo2"', 

 in diameter, the finest being pale-bordered, and probably non-me- 

 dullated • also grey substance, composed of a finely granular matter, 

 and cells of 0"00/'" to 0008'" in diameter. These cells and other 

 smaller ones, down to 0003"' in size, many of them having branched 

 processes, form the bulb of the olfactory nerve, intermingled with 

 many fine fibres, whose relation to the cells and to the proper 

 nerves of smell cannot be ascertained. The nervus opticus, with 

 its tract divided into two crura, proceeds from the corpora geni- 

 culata, the corpora quadrigemina, and the optic thalami, and is, 

 besides, connected with the cerebral peduncles, the substantia per- 

 forata antica, the tuber cinereum, and the lamina terminalis. 

 Whence its fibres, which are dark-bordered tubes, of 0*002'" in 

 diameter, really arise, is, in man, unknown (judging from experi- 

 ments on animals, they would seem principally to come from the 

 corpora quadrigemina) ; on the other hand, we know that a part 

 of them decussates in the chiasma. In the chiasma we find, 

 moreover, as stated by various authorities, 1, fibres which do not 



