THE STRUCTURE OF THE BRAIN STEM , 77 



bodies. These may be regarded as special outgrowths of the grey ma 

 of the optic thalamus, one of which, the external geniculate body is in close 

 connection with the fibres from the optic tracts, while the other, the internal 

 geniculate body, receives fibres from the lateral fillet ultimately derived from 

 the organ of hearing. In a section through the fore part of the mid-brain 

 (Fig. 190) these two bodies maybe seen lying to the outer side of the anterior 

 corpora quadrigemina, so that the fore-brain, to a certain extent, enfolds the 

 anterior part of the mid-brain. Below the thalamus at its back part is the 

 prolongation forwards of the tegmentum of the crus. This is often spoken 

 of as the subthalamic region. The red nucleus is a conspicuous object in 



e.gr.8 



FIG. 190. Transverse section through upper part of mid-brain. 

 Th, thalamus ; brs, brachium superior ; cqs, anterior (or superior) corpus 

 quadrigeminum ; cgi, cge, internal and external geniculate bodies ; /, fillet ; s, aque- 

 duct ; pi, posterior longitudinal bundle ; r, raphe ; ///, third nerve ; nlll, its 

 nucleus ; Ipp, posterior perforated space ; sn, substantia nigra ; cr, crusta ; //, 

 optic tract ; M, medullary centre of the hemisphere ; nc, nucleus caudatus ; .sV. 

 stria terminalis. 



sections through the back part of this region, but gradually diminishes as we 

 proceed forwards, and disappears before the level of the corpora mammillaria 

 is reached. The mesial fillet, which in the mid-brain lies on the lateral and 

 dorsal aspect of the red nucleus, is prolonged upwards together with fibres 

 from the superior cerebellar peduncle into the ventral part of the thalamus, 

 where probably all of the fibres end in connection with the thalamic cells. 

 The substantia nigra gradually disappears. Before it has disappeared we 

 may see on its outer side a special collection of grey matter called the nucleus 

 of Luys or the corpus subthalamicum. In addition to the anterior and 

 posterior commissures already described as connecting the two optic thalami 

 at the front and back of the third ventricle, the two sides are connected about 

 the middle of the cavity by the middle or soft commissure. The optic 

 thalamus is often described together with the corpus striatum as forming 

 the basal ganglia. The corpus striatum is, however, genetically, and 



