10-4 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



the red nucleus apparently becomes the upper lemniscus. This 

 is clearly denned and medullated in a foetus of seven months. 

 At that time the thalamus contains no medullary fibres except 

 the bundle of Vicq d'Azyr, and the internal capsule none except 

 the tegmental radiation. 



Fibres pass from the thalamus to the red nucleus, laminae 

 medullares thalami, while others go to the corpus subthalamicum. 

 Between the pes p^dunculi and all this mass of ganglia and 

 fibres, which, taken together, are called the tegmentum, is 

 situated the substantia nigra. 



We now pass into the region of the mid-brain. To this 

 region belongs (ombryologically considered) that thick bundle of 

 white fibres which passes across the ventricle at the point where it 

 becomes the aqueduct of Sylvius ; that is to say, the posterior 

 commissure (Fig. 59). It can be more easily demonstrated in the 

 lower vertebrates than in mammals that this commissure origi- 

 nates in ganglia which lie on each side, deep in the inter-brain, 

 near the median line. But, even in mammals, Meynert has 

 shown that its bundles are evolved from the thalamic nuclei. 

 They then pass dorsad, reach the surface, and cross to the oppo- 

 site side, in front of the corpora quadrigemina. They run 

 horizontally only a very short distance, and then dip down into 

 the depths of the mid-brain tegmentum, in which they pass still 

 farther caudad. The majority of the fibres under consideration 

 (as I have seen with peculiar distinctness in the lower animals) 

 pass laterally and ventrad of the posterior longitudinal fascic- 

 ulus, into the medulla oblongata. The last-mentioned fascic- 

 ulus, which we shall study later on, only attains a considerable 

 size after receiving these fibres from the commissura posterior. 

 Spitzka and Darkschewitsch have seen a similar condition in 

 mammals. According to the views of the latter, the more 

 median of these fibres pass into the nucleus of the oculo-motor 

 nerve. Certain it is that they pass so near to it that a part of 

 them seem to terminate there. The fibres can be seen passing 

 into the nucleus from the same and from the opposite half of the 



