THE GRAY MATTER. 625 



The ventral part of the pons, or the pons proper, unlike the pes of the 

 crus, contains, mixed with the fibres, a very considerable quantity of gray 

 matter. This is fairly abundant in the front part of the pons (Fig. 136) below 

 the corpora quadrigemina, but increases even more behind this (Figs. 125, 

 135). Hence, though the pons proper is largely built up of transverse and 

 longitudinal fibres, and though it contains no compact aggregations of gray 

 matter receiving special names, it does contain scattered throughout it a very 

 large quantity of gray matter, far more, indeed, than is present in the teg- 

 mental portion ; the gray matter of the pons that is, of the pons proper 

 must be regarded as forming a very important part of the gray 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. Other Collections of Gray Matter. 



543. Of these, three deserve chief attention, and may be classed 

 together, though they differ in nature. 



T he gray matter of the corpora quadrigemina. On each side of and some- 

 what dorsal to the central gray matter of the aqueduct, which, as we have 

 seen, is well developed, especially on the ventral side, collections of gray 

 matter form the chief part of the corpora quadrigemina, both anterior and 

 posterior. 



The gray matter of the anterior corpora quadrigemina (Fig. 137, A. Q. n.} 

 is more distinctly marked off from, and separated by a wider tract of white 

 matter from, the central gray matter of the aqueduct than is that of the pos- 

 terior corpora quadrigemina (Fig. 137, nPQ} ; it is, moreover, of a different 

 nature. Indeed the two pairs of bodies have quiet different relations, are of 

 different nature, and perform different functions. 



Corpora geniculata. 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. 131, 137, 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 (Fig. 

 131, B and C, Cgl.^>, the lateral or outer corpus geniculatum, the interior of 

 which consists largely of gray matter (Fig. 137, Cgl). The median portion 

 similarly bears another like swelling occupying a more median position, the 

 median or inner corpus geniculatum (Fig. 131, A and B, Cgm), the interior 

 of which (Fig. 137, Cgm) also consists of gray matter. It 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 dis- 

 tinctly concerned in vision ; the median body is derived from the mid-brain, 

 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 connec- 

 tions and probable functions of these bodies. 



Corpus dentatum of the cerebellum. In the midst of the mass of white 

 matter which is formed in the interior of the cerebellum by the confluence 

 of the three peduncles is found (Fig. 134, C D) an area of gray matter 

 arranged, like the olivary body of the bulb, as a sharply folded or plaited 

 band in the shape of a flask or bowl. As in the similar olivary body the 

 gray matter of the flask is covered up by and its interior filled up with white 



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