444 SPECIAL HISTOLOGY. 



ways, as in the optic thalamus, from the tuberculum anterius 

 of which its radix descendens arises ; in the corpus mammillare 

 (vid. sup. § 116); at the commencement of the radix ascendens; 

 in the floor of the third ventricle, towards which some delicate 

 fasciculi of the radix ascendens are given off ; and at its point 

 of junction with the septum pellucidum, which latter, together 

 with a common thick coat presenting much connective tissue 

 and corpuscida amylacea (vid. § 118), exhibits numerous 

 plexuses of the finest kind of nerve-fibres and nerve-cells, 

 exactly as does the tuber cinereum. The fibres of the fornix 

 measure, in its white portions 0-0008— 0-005'", mostly 0-002 — 

 0-003'"; in the optic thalamus (upper part), and in the corpus 

 mammillare, the fibres are only of the finest sort, measuring 

 0-0004 — 0*001'". The comu ammonis, and the calcar avis 

 (pes hippocampi minor), present nearly the same conditions as 

 those of the cerebral convolutions ; in the grey substance of 

 the former, however, there is a peculiar sort of streak, contain- 

 ing 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 olfactorius, fine nerve-fibres, of 0-0004'", or at 

 most 0-002'", the finest, pale-bordered, and apparently non- 

 medullated ; and besides these, also some grey substance, with 

 fine granular structure, and cells of 0*007 — 0-008'". These 

 cells, with some still smaller, down to a diameter of 0-003"', 

 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 substantia perforata antica, the tuber cinereum, 

 and the lamina terminalis. The precise origin of its fibres, 

 dark-bordered tubes of 0-002'", is in Man unknown, but to 

 draw conclusions from experiments in animals, it exists princi- 

 pally 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. com- 



