638 



STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XLIV. 



lies dorsal to the fillet, but terminates like it in the subthalamic 

 region and optic thalamus, whence a new relay carries on the 

 impulses to the cortex. 



Ventral to the tegmentum is a layer of grey matter, of which 

 the cells are deeply pigmented; hence it is called the substantia 

 nigra (S.K). This receives many collaterals from the pyramidal 

 bundles. 



The white matter on the ventral side of this is known as the 

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



C.Q. 



c.o. 



L.G. 



FIG. 465. Outline of two sections across the mid -brain: A, through the middle of the inferior; B, 

 through the middle of the superior corpora quadrigemina, C.Q. Cr., crusta; 8.N., substantia nigra 

 shown only on one side ; T, tegmentum ; S, Sylvian aqueduct, with its surrounding grey matter ; 

 L.G., lateral groove; p.L, posterior longitudinal bundle; d.F., descending root of the fifth nerve; 

 S.C.P., superior cerebellar peduncle ; F, fillet ; III., third nerve. The dotted circle in B represents 

 the situation of the tegmental nucleus. In B the three divisions of the crusta are indicated on one 

 side. The pyramidal fibres (Pi/) are in the middle, and the fronto-cerebellar (F.C.) and temporo- 

 occipital cerebellar (T.O.C.) at the sides. (After Schafer.) 



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, it is believed, 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 longitudinal bundle ; this 

 gives off collaterals to Fthe nuclei of the three nerves that supply the 

 eye muscles, and then runs ventro-laterally to the posterior longi- 



