238 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the nucleus nervi vestibuli medialis ; (3) the nucleus nervi vestibuli lateralis 

 (nucleus of Deiters); and (4) the nucleus nervi vestibuli superior. Finally, 

 among other connections are to be specially mentioned those with the cere- 

 bellum (nuclei fastigii, nucleus dentatus, and the cerebellar cortex). Nothing 

 definite is known concerning the pathways by which the impulses entering 

 over the radix vestibularis reach the cortex. 



Nervus Trigemmus {Fifth Nerve). — The neurones of this nerve have their 

 cell-bodies located in the ganglion semilunari (Gasseri), and the peripheral 

 axones act as the nerves of common sensation for the skin of the face and 

 tongue and the mucous membranes of the mouth. The central axones which 

 branch on reaching the bulb send their shorter divisions eephalad for a little 

 distance, and the longer caudad, in both cases finding cells of reception in 

 the region of the substantia gelatinosa, and in the latter instance extending 

 caudad at least as far as the first segment of the spinal cord. Possibly, one 

 set of neurones of the fifth passes directly into the cerebellum. The path- 

 way from the nucleus of termination to the cortex has not been determined. 



Second Nerve, Optic. — As has long been recognized, the optic nerve, so 

 called, is a cerebral tract morphologically equivalent to such tracts as con- 

 nect any portion of the cerebral cortex with a primary centre, the retina 

 being in part the representative of the cerebrum ; and the pulvinares, the 

 quadrigemina, and geniculata externa being the primary centres. 



At the chiasma where the two optic nerves come together their fibres 

 intermingle, and then emerge as the optic tracts, which contain not only the 

 fibres connected with the retina, but others added from the superposed parts 

 of the brain, and forming the commissures of Meynert and von Gudden. 



In the rabbit it was shown by von Gudden l that in the chiasma the 

 majority of the fibres forming one optic nerve pass to the tract of the 

 opposite side, but that a portion of the fibres remains in the tract of the 

 same side. 



This was inferred because removal of one eyeball caused in young rab- 

 bits a degeneration in the associated optic nerve and also in both optic tracts — 

 most marked, however, in the tract of the side opposite to the lesion. Con- 

 versely, the section of one optic tract causes a degeneration in both optic 

 Derves, the nerve of the side opposite to the lesion being most affected, and a 

 smaller degeneration appearing in the nerve of the same side (sec Fig. 98). 



In the fish, amphibia, reptiles, and birds — except the owls 2 — the decussa- 

 tion appears to be complete. 3 For the partial decussation in the owls the 

 evidence is physiological. This distribution of the optic fibres was associated 

 by von (bidden with the position of the eyes in the head. The extreme 

 lateral position of the eyes as it occurs in the lower mammals permits of but 

 little combination of the two visual fields; whereas the position in man, in a 



Won Gudden : Qesammelte wnd hinterlassene Abhandlungen, Wiesbaden, 1889. 

 2 Fcrrier : The Croonian Lectures on Cerebral Localization, London, 1890, p. 70. 

 "' Singer and Munzer : IJenkschriJ'ten der math.-naturwiss. Classe der kais. Akademie der Wixscn- 

 schaften, 1888, Bd. iv. 



