XI THE BRAIN 509 



cerebi'i, but lying somewhat in front; of the optic thalami. 

 Hence the corpora striata are seen as a projection on the 

 floor of the lateral ventricles (Fig. 163, G.S., C.S.), and as 

 part of the side wall of tlie front end of this ventricle 

 (Fig. ]61, NC). Each corpus consists of two parts, one 

 lying in front of the optic thalamus, the other further 

 back and by the side of the thalamus. Tlie larger part of 

 each corpus striatum is imbedded in the neighbouring 

 substance of the cerebral hemisphere, with which it is 

 intimately connected by nerve fibres. It is also similarly 

 connected with the fibres of the crus on which it lies. 



A clear understanding of the position and relations of 

 the optic thalami and corpora striata is essential in con- 

 nection with the course of a tract of nerve fibres with 

 which we shall deal later, known as the internal 

 capsule. 



The Membranes of the Brain. — The brain is invested 

 by three membranes whicli are the same in name, and 

 similarly placed and related to each other as tliose which 

 we have previously described as covering the spinal 

 cord (see p. 453). Of these the pia mater is highly 

 vascular, and carries blood-vessels down into the grey 

 matter, especially in the sulci or grooves to which 

 the convoluted appearance of the surface of the brain 

 is due. Moreover, it forms a roof to the hinder 

 part of the cavity of the fourth ventricle, and a highly 

 developed layer of the pia mater is tucked in under the 

 hinder end of the cerebral hemispheres to form the roof 

 of the third ventricle ; this is known as the velum 

 interpositum. The edges of this velum as it lies 

 beneath the fornix project on each side into the cavities 

 of the lateral ventricles arid are here known as the 

 choroid plexuses, the whole being arranged with a 

 view to the nutrition of the internal parts of the 

 brain. The cavities of the cerebral ventricles, and 

 hence of the central canal of the spinal cord, are 

 placed in communication with the subarachnoid space, by 



