982 INTERMEDIATE GREY MATTER. [BOOK in. 



ventral to the hind part of the optic thalamus a collection of grey 

 matter called the corpus subthalamicum (Fig. 116, C.sb). 



At the hinder part of the crus, as it is about to plunge into 

 the pons, while the pes, now decreasing relatively in size, still con- 

 tinues to be ordinary white matter composed of longitudinal 

 bundles of medullated fibres, the tegmentum takes on more and 

 more the structure which in speaking of the bulb we called 

 reticular formation, and which, as we saw, deserves to be con- 

 sidered as a kind of grey matter. 



The grey matter of the pons. When the conjoined crura as we 

 trace them backward plunge beneath the pons the longitudinal 

 fibres of the pes of each crus are as we have said soon split up 

 into bundles scattered among the transverse fibres belonging to 

 the pons itself. Dorsal to this system of transverse and longi- 

 tudinal fibres forming the pons proper, between it on the ventral 

 surface and the central grey matter with the posterior corpora 

 quadrigemina on the dorsal surface, is a region which may be 

 called tegmental since it is a continuation of the tegmentum of 

 the crus. In the front part of the pons (Fig. 113), where the 

 posterior corpora quadrigemina still form the dorsal roof of the 

 section, this tegmental area, which is much broken up by certain 

 strands of longitudinal fibres of which we shall speak later on, 

 contains scattered nerve cells, and is largely composed of reticular 

 formation. In this is placed on each side a group of nerve cells, 

 the locus caeruleus (Fig. 113, I.e.), to which we have already 

 referred (621) as probably serving in part as the origin of the 

 descending root of the fifth nerve (V. d.), just ventral to which 

 it lies. This acquires larger dimensions farther back, in the 

 front part of the fourth ventricle (Fig. 115, I.e.) between the levels 

 represented in Figs. 112 and 113, and is a collection of large 

 spindle-shaped nerve cells; it has a bluish tint when its black 

 pigment is seen shining through the surrounding more or less 

 transparent material, hence the name. 



In the hinder parts of the pons (Figs. Ill, 112) where the 

 cerebellum is seen overhanging the open fourth ventricle, the 

 reticular formation of the tegmental area is still more conspicuous. 

 The only special collection of grey matter in this region to which 

 we need call attention is one which, consisting like the olivary 

 body of the bulb (or inferior olive) of a wall of grey matter 

 surrounding and surrounded by white matter, is called the upper 

 olive (Figs. Ill, 112, s.o.). 



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 grey matter. This is fairly abundant in the front part 

 of the pons (Fig. 113) below the corpora quadrigemina but in- 

 creases even more behind this (Figs. Ill, 112). Hence though the 

 pons proper is largely built up of transverse and longitudinal 

 fibres, and though it contains no compact aggregations of grey 



