CH. XLVI.] ORIGINS AND FUNCTIONS OF CRANIAL NERVES 673 



crusta (Or) orpes. It is here that the pyramidal bundles are situated ; 

 these occupy its middle three-fifths (Py). The mesial fifth is occupied 

 by fibres passing from the frontal region of the cerebrum to the pons, 

 and thence to the cerebellum ; hence they are called fronto-cerebellar 

 fibres. The fibres occupying the lateral fifth are usually spoken of 

 as temporo-occipital cerebellar fibres, but there is no certainty as yet 

 regarding their origin or functions. 



The corpora quadrigemina are formed mainly of grey matter; 

 from each superior corpus a bundle of white fibres passes upwards 

 and forwards to the geniculate bodies, eventually joining the optic 

 tract of the same side. The white layer on the surface of the grey 

 matter of the C. quadrigemina is derived from the optic tract; these 

 fibres come from the retina, and terminate by arborising around the 

 cells of the grey matter of the C. quadrigemina. 



The cells of the grey matter of the corpora quadrigemina differ 

 greatly in form and size ; the destination of their axons is not pre- 

 cisely known, but some pass ventralwards, cross at the raphe, and 

 constitute the fountain decussation of Meynert ; after decussation 

 they form the main mass of the ventral longi- 

 tudinal bundle ; this gives off collaterals to the 

 nuclei of the three nerves that supply the eye 

 muscles, and then runs ventro-laterally to the 

 posterior longitudinal bundle, with which its 

 fibres ultimately mix in the antero-lateral 

 descending tract of the spinal cord. Fio 415 ._ Sect ^ through 



Seventh section. This is through the cms of cerebrum, cr, 



T , j e .1 / !_ i_ j. crusta ; S.N.. substantia 



CrUS. It IS made lip Of CrUSta (which Contains nigra; T, tegmentum. 



the motor fibres), tegmentum (which contains 



the sensory fibres, especially the bundle called the mesial fillet), and 



the substantia nigra, the grey matter which separates them. 



Origins and Functions of the Cranial Nerves. 



Having now studied the internal construction of these parts, we 

 can take up more fully the origins and functions of the cranial nerves 

 which originate there. The olfactory nerve is connected to the 

 cerebrum, and will be considered with the sense of smell. The 

 optic nerve will be studied with vision, though it is, as we have seen, 

 immediately connected with the mid-brain. 



The third, fourth, and sixth nerves supply the muscles of the eye. 

 Gaskell discovered among the rootlets of the third and fourth nerves 

 the vestiges of a degenerated and functionless ganglion, which indicates 

 the previous existence of a separate sensory root. Sherrington has 

 shown that in these three nerves sensory fibres are present which are 

 connected to the sensorial nerve endings (muscle- spindles). 



The third nerve (motor oculi) arises in a group of nerve-cells in 



2 U 



