CHAP, ii.] THE BRAIN. 971 



and endings; but these all contribute to the crura and are 

 so far alike as to justify us in considering them as a system. 

 The cortical grey matter of each hemisphere is, as we shall see, 

 connected with various parts of this system, and in one sense 

 we may regard this system as beginning in the cortex of each 

 hemisphere, and ending in the spinal cord. But certain masses 

 of grey matter in the hemisphere not strictly cortical, and several 

 important masses and areas of grey matter lying between the 

 hemisphere and the cord, are connected with the system ; and 

 these we may speak of as the " intermediate grey matter of the 

 crural system." 



Corpus striatum and optic thalamus. Of all these several 

 collections of grey matter, the largest, most conspicuous, and 

 perhaps the most important are the two masses in the front 

 part of the system known as the corpus striatum and optic 

 thalamus. The former is, as we have seen ( 602), a development 

 of the wall of the cerebral vesicle, the latter a development of 

 the wall of the vesicle of the third ventricle. They are therefore 

 of different origin ; although in the course of the growth of the 

 brain they become closely attached to each other, they are at the 

 outset quite separate and distinct. Moreover, as we shall see, 

 they differ from each other so essentially, in their nature and 

 relations, that they cannot be considered as homologous bodies ; 

 and the term " basal ganglia " often applied to them is therefore 

 unfortunate. Nevertheless it will render the description of their 

 topographical relations easier, if for a little while we consider 

 them together. 



When the lateral ventricle is laid open from above, part of the 

 corpus striatum is seen projecting into the cavity of the ventricle. 

 In front the projecting part is broad, forming the lateral wall and 

 part of the floor of the ventricle, and to its median side lies the 

 cavity of the ventricle, separated from its fellow by the septum 

 lucidum. Farther back the projecting part, becoming gradually 

 narrower, assumes a more lateral position and passes into the 

 descending horn. In this part of its course there lies on its 

 median side, separated from it by a narrow band called the 

 tsenia semicircularis or stria terminalis, the optic thalamus, a 

 narrow 7 strip of the surface of which is seen projecting outside 

 the edge of the choroid plexus. If now, not only both lateral 

 ventricles be laid open by removal of the corpus callosum and 

 the fornix with the velum interpositum and choroid plexus be 

 taken away, so as fully to expose the third ventricle, but also, 

 in order to obtain a better view, the whole of the hinder part 

 of the cerebrum containing the posterior horns of the lateral 

 ventricle, be completely cut away, it is seen (Fig. 115) that the 

 two optic thalami (0. T.) present themselves as two large oval 

 bodies, placed obliquely athwart the diverging crura cerebri and 

 converging in front to form the immediate walls of the third 



