890 SPECIAL HISTOLOGY. 



avis (pcs hippocampi minor), present nearly the same conditions as 

 those of the cerebral convolutions ; in the gray substance of the former, 

 however, there is a peculiar sort of streak, containing chiefly round cells 

 without processes, and closely aggregated. 



Lastly, we have to consider the origin of the first two pairs of nerves. 

 The olfactory nerve contains, in the white portion of the tractus olfacto- 

 rius, fine nerve-fibres, of 0-0004, or at most 0-002 of a line, the finest, 

 pale-bordered, and apparently non-medullated ; and besides these, also 

 some gray substance, with fine granular structure, and cells of 0-007- 

 0-008 of a line. These cells, with some still smaller, down to a diameter 

 of 0-003 of a line, many with branched processes, constitute the 

 bulbus n. olfactorii, intermixed with numerous fine fibres, the relation of 

 which to the cells and to the true nerves of smell cannot be made out. 

 The optic nerve arises, with its tractus divided into two crura, from the 

 corpora geniculata, and the corpora quadrigemina and optic thalami ; 

 besides which, it is also in connection with the crura cerebri, the sub- 

 stantia perforata antica, the tuber cinereum, and the lamina terminalis. 

 The precise origin of its fibres, dark-bordered tubes of 0-002 of a line, 

 is in Man unknown, but to draw conclusions from experiments in ani- 

 mals, it exists principally in the corpora quadrigemina, whilst we know 

 that they partially decussate in the chiasma (commissure). In this body, 

 however, there are, as stated by Arnold, Todd and Bowman, &c. : 1, 

 fibres which do not decussate, but are continued from the tractus into 

 the optic nerve of the same side ; and 2, commissural fibres, which may 

 indeed be divided into an anterior and posterior set, the latter constituting 

 a commissure between the two points of origin of the optic nerves, whilst 

 the anterior could only unite the two retince. The existence of the 

 first-named fibres is certain, although they are much more scanty than 

 the decussating elements ; but that of the others also can hardly be 

 denied. Speaking physiologically, a commissure of the optic thalami 

 and corpora quadrigemina may perhaps be explained, but a commissure 

 also of the retince does not appear to be altogether impossible, because 

 we know that the retina contains gray substance, and in it, nerve-cells 

 with branched processes. 



"With respect to the origin of the nerve-fibres in the brain and higher 

 central organs, in general, it is several years since I first observed the 

 origin of dark-bordered fine fibres from the processes of the nerve-cells 

 in the spinal cord of the Frog ("Zeitsch. f. wiss. Zool.," vol. I. p. 144, 

 tab. xi. Fig. 7). In man I have not as yet been so fortunate as to per- 

 ceive anything of the sort with certainty, though I do not myself doubt 

 that similar conditions obtain in this case also. In fact, R. Wagner and 

 Leuckart think they have seen, in man, the processes of the many-rayed 

 cells in the substantia ferruginea, passing into nerve-tubes (" Gott. An- 



