THE STRUCTURES AT THE BASE OF THE BRAIN. 89 



tegmcntal tract to regions farther back (dorsad of the gray mass 

 marked " corpus subthal.") is not visible because it does not fall 

 within the plane of this section. Compare the line in Fig. 52, 

 marked "to the fillet," which represents the course of this 

 bundle. Compare also Fig. 54. 



You have now. gentlemen, learned the origin and the first 

 part of the course of a good share of those fibres which go to 

 make up the fore-brain. Let us now turn our attention to the 

 region where a majority of the cerebral medullary tracts 

 terminate. 



Back of the hemispheres comes the inter-brain. From 

 each of its lateral walls have developed the optic thalami. These 

 consist of several gray nuclei, which are not sharply divided from 

 one another. Some white medullary fibres, the stratum zonale, 

 pass over the thalamus. A part of these can be traced in a 

 direction toward the base of the brain in the optic nerve, while 

 another part seems to originate from the caudal portion of the 

 internal capsule, perhaps from the optic radiation. All of them 

 pass deeply into the thalamus and are massed together between 

 its ganglia, which they apparently separate from one another. 

 Microscopic examination shows that they enter into the fine net- 

 work of nerve-fibres which penetrates these ganglia. We can 

 distinguish in every thalamus a median (internal) nucleus which 

 projects into the ventricle, becomes thickened posteriorly, and 

 forms the pulvinar, a lateral or external nucleus, and between 

 the two an anterior nucleus. The lateral nucleus is the largest, 

 while the anterior nucleus may be compared to a wedge driven 

 in between the other two with its broad end to the front, This 

 anterior, thickened end, which is visible as an elevation of the 

 surface of the thalamus, we have already met with under the 



ti 



name of tuberculum anterius. On the median border of the 

 internal nucleus lies the ganglion habenulse, which- has been 

 mentioned before. In the posterior portion of the thalamus 

 below and external to the pulvinar lies a ganglion of a peculiar 

 grayish appearance, the corpus geniculatum laterale; it projects 



