THE NERVOUS SYSTEM. 615 



also of others which end in the masses of gray matter in the pons or crus 

 itself; but the function of all of the fibres is believed to be to carry im- 

 pulses downward from the cerebrum either to the spinal cord and so to 

 the cranial nerves, or to the cerebellum. 



The tegmentum of either side, on the other hand, is supposed to be 

 concerned, for the most part at any rate, with afferent impulses. It is 

 made up to a very considerable extent of collections of gray matter, the 

 most important of which are (a) the locus or nucleus niger, separating 

 the pes and tegmentum ; (b) the nucleus ruber, which is a rounded mass 

 situated more toward the aqueduct of Sylvius; this ' extends from the 

 third ventricle to the anterior corpus quadrigeminum. The locus niger 

 extends back as far as the posterior corpus quadrigeminum. (c) A third 

 mass of gray matter is situated beneath the optic thalamus, and is the 

 corpus subthalamicum. Posteriorly the tegmentum is made up chiefly 

 of the reticular material so often spoken of, and in the pons consists 

 almost entirely of that kind of structure, but with the two additional 

 masess of gray matter already indicated, viz., the locus coeruleus and 

 superior olive. 



It will be as well here to indicate briefly the other collections of gray 

 matter in the neighborhood of the crura, viz., the corpus striata, optic 

 thalami, corpora quadrigemina, corpora geuiculata, and the corpora 

 dentata of the cerebellum. 



Corpora Striata. The corpora striata are situated in front and to 

 the outside of the optic thalami, partly within and partly without the 

 lateral ventricle. 



Each corpus striatum consists of two parts : 



(a.) An intraventricular portion (caudate nucleus) which is conical in 

 shape, with the base of the cone forward ; it consists of gray matter, 

 with white substance in its centre, (b.) An extraventricular portion 

 (lenticular nucleus), which is separated from the other portion by a layer 

 of white material, which forms a portion of the internal capsule, the 

 anterior limb. The lenticular nucleus is seen, on a horizontal section of 

 the hemisphere, to consist of three parts- (the two internal called globus 

 pallidus, major and minor, and the outer called the putamen), separated 

 from one another by white matter, of which the smallest of the three is 

 inside. Each part somewhat resembles a wedge in shape. The upper 

 and internal surface is in relation with the caudate nucleus, being sepa- 

 rated from it by the anterior limb of the internal capsule. The remain- 

 der of the internal surface is in relation to the optic thalamus, being 

 separated from it by the posterior limb of the internal capsule. The 

 horizontal section is wider in the centre than at the ends. On the out- 

 side is the gray lamina (claustrum) separated by a thin white layer 

 external capsule from the lenticular nucleus. 



