THE STRUCTURES AT THE BASE OF THE BRAIN. 95 



the tract called the fillet without entering into any relation with 

 the interior of the corpus subthalamicum. 



Fibres from the optic thalamus run to the red nucleus. 

 Posterior to the accompanying figure the latter becomes much 

 larger, so that it occupies a greater area in cross-section (Fig. 59). 

 Posterior to and below the corpus subthalamicum, just over the 

 fibres of the internal capsule, which at this point are beginning 

 to form the pes pedunculi, there is situated an aggregation of 

 gray, pigmented cells (the substantia nigra), just at the point 

 where the corpus subthalamicum is represented in Fig. 54. 

 From the subthalamic region downward to the termination of 

 the mid-brain this dark, ashen-gray ganglion is always demon- 

 strable just over the crusta. 



The fibres passing backward from the fore- and inter- brains 

 are divided by the substantia nigra into two portions, corre- 

 sponding to their physiological significance, the crusta and the 

 tegmentum. The former we have already considered at the 

 beginning of this lecture, and we shall often have occasion to 

 refer to it; the latter contains, in the posterior thalamic region, 

 which we are now considering, the pulvinar, the nucleus ruber, 

 the corpus subthalamicum, the fibres from the lenticular nucleus 

 and that part of the tegmental radiation which has not united 

 with the fibres of the lenticular nucleus. 



We are now to consider the structures which appear in a 

 transverse section made at a point in Fig. 55 indicated by the 

 line a to 6. 



You see that just back of it the mid-brain the corpora 

 quadrigemina begins. The thalami at this point become sep- 

 arated and between them the central gray matter increases some- 

 what, and the middle ventricle thereby becomes much more 

 shallow. 



Behind this point the roof of the brain-vesicles, which, in 

 the vicinity of the thalamus, consisted only of the epithelial 

 covering of the choroid plexus, once more becomes distinct. 

 The ventricle is closed above by a roof composed of nerve- 



