624 THE BRAIN. 



along its course. But, besides these and the nuclei of the third and fourth 

 cranial nerves, of which we have already spoken, certain larger collections 

 of gray matter deserve attention. A conspicuous mass of gray matter, cir- 

 cular in transverse section, placed in the midst of the tegmentum on each 

 side but somewhat near the middle line, and stretching from the hinder 

 margin of the third ventricle beneath the anterior corpus quadrigeminum 

 (Figs. 137, 138), is, from the red tint it possesses, called the red nucleus, 

 nucleus, or locus ruber. It is traversed by fibres of the third nerve as these 

 make their way ventrally from the nucleus to the surface. 



We must consider also as belonging to the tegmentum a large area of 

 gray matter, somewhat lens-shaped in section (Fig. 137, Sn), which lies 

 between the pes and tegmentum, sharply marking off the one from the other. 

 From its dark appearance, due to the abundance of black pigment, it is 

 called the substantia nigra or locus niger. It acquires its largest dimensions 

 at about the middle of the length of the crus, coming to an end in front 

 (Fig. 139, Sn) and fading away behind (Fig. 136), as the crus passes beneath 

 the posterior corpora quadrigemina. These two, the red nucleus and the 

 substantia nigra, are perhaps the most important collections of gray matter 

 in the tegmentum, but we may add that at the front of the crus as the sub- 

 stantia nigra comes to an end there is seen in a somewhat similar position 

 ventral to the hind part of the optic thalamus a collection of gray matter 

 called the corpus subthalamicum (Fig. 139, 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 continues 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 gray matter. 



The gray 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 and scattered among the 

 transverse fibres belonging to the pons itself. Dorsal to this system of trans- 

 verse and longitudinal fibres forming the pons proper, between it on the ven- 

 tral surface and the central gray matter with the posterior corpora quadri- 

 gemina 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. 136), 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 cceruleus (Fig. 

 136, I. c.), to which we have already referred ( 534) as probably serving in 

 part as the origin of the descending root of the fifth nerve ( V. d.), just ven- 

 tral to which it lies. This acquires larger dimensions further back, in the 

 front part of the fourth ventricle (Fig. 138, /. c.) between the levels repre- 

 sented in Figs. 135 and 136, 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. 134, 135), 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 

 gray matter in this region to which we need call attention is one which, con- 

 sisting, like the olivary body of the bulb (or inferior olive), of a wall of 

 gray matter surrounding and surrounded by white matter, is called the 

 upper olive (Figs. 134, 135, s. o.). 



