THE STRUCTURES AT THE BASE OF THE BRAIN. 91 



From the fact that the majority of the medullary fibres pass into 

 the external nucleus, this body has a somewhat lighter appearance 

 than the other nuclei of the thalamus. 



The inner surface of the thalamus is separated from the 

 ventricle by an evenly distributed layer of gray matter. This 

 is called the central gray matter of the middle (third) ventricle. 

 In the median line of the brain the central gray matter forms 

 the floor of the ventricle. Between the two thalami lies the 

 middle commissure. In human beings it consists of only a few 

 nerve-fibres passing across in the midst of a large mass of cen- 

 tral gray matter. In the lower animals it contains a relatively 

 larger number of medullary fibres, arising in the central gray 

 matter of the ventricles, in which they run parallel to the long- 

 axis of the brain. 



In the following highly diagrammatic figure we may see the 

 relations of the thalamus to the base of the brain, to the central 

 gray matter, to the internal capsule, and to the nucleus lenti- 

 formis. 



Let us study in this figure a subject which hitherto has only 

 been touched upon. I refer to the region internal to the len- 

 ticular nucleus and ventrad of the thalamus. Here are to be 

 seen several bundles of fibres running parallel with each other, 

 which in part cross the internal capsule at an angle and in part 

 pass over it. The upper of these bundles belongs to the system 

 of fibres of the lenticular nucleus; it is the previously-described 

 ansa lenticularis. The lower bundles are coronal fibres to the 

 thalamus, w r hich, coming from the occipital and temporal lobes, 

 are called the inferior pedicle of the thalamus (?.., Fig. 43). 

 Taken together the mass of fibres shown in Fig. 52, ventrad of 

 the lenticular nucleus, is called the substantia innominata. Just 

 back of the substantia innominata the fibres of the capsule, 

 which go to make up the crusta, emerge free on the base of the 

 brain. The substantia innominata, therefore, bounds the pedun- 

 culus cerebri anteriorly. It is laid like a loop over it, and is on 

 that account called the ansa peduncularis. 



