374 ELEMENTARY ANATOMY. [LESS. 



Two rounded bodies (the optic thalami} are placed one on 

 each side of the first described cavity (the third ventricle], 

 and are connected by the soft and posterior commissures. 

 Two other rounded bodies (the corpora striata) are placed one 

 in each cerebral hemisphere between the anterior and descend- 

 ing cornua. They are connected by the anterior commissure. 



FIG. 329. DIAGRAM OF A TRANSVERSE VERTICAL SECTION OF THE BRAIN, 

 MADE THROUGH THE SECOND, THIRD, AND FlFTH VENTRICLES. 



f, cerebrum ; c', its temporal lobe : cc, corpus callosum ; cm, corpora mammillaria; 

 _/", fornix ; fm, foramen of Monro ; op, one of the two optic thalami ; pt, the 

 pituitary body ; s, middle, or soft commissure ; st, one of the two corpora 

 striata ; v, velum interpositum ; x, space enclosed between the velum and the 

 fornix ; y, fissure of Sylvius ; 2, lateral ventricles ; 2', the ascending cornu of 

 a lateral ventricle; 2' , its descending cornu ; 3 and 3', the third ventricle ; 5, 

 the fifth ventricle. 



8. The DEVELOPMENT of this complex organ is as follows : 

 At first there are three hollow vesicles placed one in front of 

 the other, their three cavities (which open one into another) 

 being expansions of the anterior end of the primitive groove 

 ind subsequent canal of the embryonic cerebro-spinal axis. 

 These three vesicles are called respectively, (i) the hind- 

 brain, (2) the mid-brain, and (3) the fore-brain (Fig. 330). 



The superior surface of the fore-brain becomes the velum 

 interpositum, and from its hinder part the pineal gland 

 arises, while the infundibulum and pituitary body appear at 

 its inferior surface. Its cavity is therefore what afterwards 

 becomes the third ventricle. 



The mid-brain becomes the corpora quadrigemina above 

 and the crura cerebri below, while its greatly reduced cavity 

 is ultimately the iter a tertio ad quartum ventriculum. 



