588 THE BRAIN. 



brain. Meanwhile we may point out, that while this vascular ingrowth seems 

 to make the cavity of the third ventricle continuous with that of the lateral 

 ventricle on each side, and all three with the exterior of the brain, it really 

 does not do so. The cavity of the third ventricle is made complete by the 

 layer of epithelium forming its roof, and the cavity of the lateral ventricle 

 is made complete by the layer of epithelium passing from the lateral edge of 

 the fornix over the choroid plexus to the other parts of the wall of the 

 ventricle. To pass along this line from the actual cavity of the lateral into 

 that of the third ventricle one must first pierce the epithelium covering the 

 choroid plexus, thus gaining access to. the pia mater of the plexus and of the 

 velum, and then again pierce the epithelium coating the under surface of 

 the vellum and forming the roof of the third ventricle It is only by the 

 foramen of Munro that a real communication exists between the cavity of 

 the lateral and that of the third ventricle. 



Thus by the large growth and backward extension of the cerebral hemi- 

 spheres, the third ventricle comes to form, as it were the front end of the 

 cerebro-spinal axis, the crura cerebri expanding on each side of the third 

 ventricle into the cerebral hemispheres which cover up the ventricle on the 

 dorsal surface, but leave its wall exposed on the ventral surface. Attached 

 to the dorsal surface of the third ventricle at its hind end, ventral to and 

 somewhat projecting beyond the base of the fornix, lies the pineal gland 

 with its attachments, the remnants of a once-important median organ ; and 

 attached to the ventral surface of the ventricle, at the apex of a funnel- 

 shaped projection, the infundibulum, lies the pituitary body, also a remnant 

 of important ancestral structures. 



516. We may then divide the whole brain into a series of parts corre- 

 sponding to the main divisions of the embryonic brain. At the front lie 

 the cerebral hemispheres, with the lateral ventricles, developed out of the 

 cerebral vesicles ; and with these are associated the corpora striata, the term 

 cerebral hemisphere being sometimes used so as to include these bodies, and 

 sometimes so as to exclude them. Next come, corresponding to the original 

 fore-brain, the parts forming the walls of the third ventricle, conspicuous 

 among which are the optic thalami ; for these bodies, though they appear 

 to intrude into the lateral ventricles, belong properly to the third ventricle. 

 In the mid-brain which follows, the cavity, now the tubular passage of the 

 aqueduct, is roofed in by the two pairs, anterior and posterior, or corpora 

 quadrigemina, the dimensions of which are not very great ; but a thick 

 floor is furnished by the crura cerebri. In each crus we must distinguish 

 between a dorsal portion called the tegmentum, in which a large quantity 

 of gray matter is present, and in which a great complexity in the arrange- 

 ment of fibres exists, and a ventral portion, the pes or crusta, which is a 

 much more uniform mass of longitudinally disposed fibres. As the crura 

 passing forward diverge into the cerebral hemisphere on each side, the teg- 

 mentum ceases at the hinder end and ventral parts of the optic thalamus ; 

 it is the pes which supplies the mass of fibres radiating into each cerebral 

 hemisphere. In a view of the ventral surface of the brain, the base of the 

 brain as it is frequently called, the crura may be seen emerging from the 

 anterior border of the pons. This we have spoken of as the thickened floor 

 of the front part of the hind-brain, but in reality it encroaches a little on 

 the mid-brain, the hind part of the corpora quadrigemina being in the same 

 dorso-ventral plane as the front part of the pons. (See Fig. 131.) In the 

 main, however, the pons belongs to the fore part of the hind-brain, the roof 

 and sides of which are developed, as we have said, into the cerebellum. 

 This superficially resembles the cerebral hemispheres in its large size, and 

 in the special development of its surface, which is formed of gray matter 



