CHAP. IL] THE BRAIN. 983 



matter receiving special names, it does contain scattered through- 

 out it a very large quantity of grey matter, far more indeed than 

 is present in the tegmental portion ; the grey matter of the pons, 

 that is of the pons proper, must be regarded as forming a very 

 important part of the grey matter of the crural system, and of 

 no little physiological significance. 



Behind the pons the crural system is continued into the bulb, 

 with whose structure we have already dealt. 



4. Oilier Collections of Grey Matter. 



630. Of these, three deserve chief attention, and may be 

 classed together, though they differ in nature. 



The ([rey matter of the corpora quadrigemina. On each side of 

 and somewhat dorsal to the central grey matter of the aqueduct 

 which, as we have seen, is well developed especially 011 the ventral 

 side, collections of grey matter form the chief part of the corpora 

 quadrigemina, both anterior and posterior. 



The grey matter of the anterior corpora quadrigemiua (Fig. 

 114, A. Q. n) is more distinctly marked off from, and separated by 

 a wider tract of white matter from the central grey matter of the 

 aqueduct than is that of the posterior corpora quadrigemina (Fig. 



113, nPQ): it is moreover of a different nature. Indeed the two 

 pairs of bodies have quite different relations, are of different 

 nature, and perform different functions. 



Corpora c/eniculata. The two optic nerves, as we shall see in 

 detail later on, give rise, through the optic decussation, to the two 

 optic tracts. Each optic tract (Figs. 108, 114, Op), winds round 

 the crus cerebri on its ventral surface to reach the substance of 

 the hemisphere in the region below the optic thalamus, and as it 

 does so is described as dividing into a lateral and median portion. 

 The lateral portion just as it sweeps round the far edge, that is 

 the outer or lateral edge, of the crus bears a rounded swelling 

 (Figs. 108 B and C, C.gl.) the lateral or outer corpus genicu- 

 latum, the interior of which consists largely of grey matter (Fig. 



114, Cgl). The median portion similarly bears another like 

 swelling occupying a more median position, the median or inner 

 corpus geniculatum (Fig. 108 A and B, Cgni), the interior of 

 which (Fig. 114, Cgm] also consists of grey matter. Xlt is to be 

 regretted that these two bodies should bear the same name, for 

 they are different in their origin, in their connections, and in their 

 functions. The lateral body is said to be derived from the fore- 

 brain, that is from the vesicle of the third ventricle, has definite 

 connections with the retinal optic fibres, and is distinctly con- 

 cerned in vision ; the median body is derived from the midbrain, 

 is not definitely connected with the retinal fibres, and appears to 

 be in no way concerned in vision. We shall however return later 

 on to the connections and probable functions of these bodies. 



