THE DIENCEPHALON. 1131 



callosum. Just before meeting the latter, the lamina passes in front of the anterior 

 commissure of the brain (Fig. 976). 



The Third Ventricle. — The third ventricle (ventriculus t;rtius cerebri) is the 

 narrow cleft-like space that separates the medial surfaces of the thalami (Fig. 966). 

 It is somewhat broader behind and much deeper in front, where it comes into close 

 relation with the exterior of the brain, the interpeduncular lamina alone intervening. 

 Seen from the side, as in mesial sagittal sections (Fig. 996), the outline of the 

 ventricle is irregularly comet-shaped, with the broader end above and behind and the 

 blunted point directed downward and forward (Fig. 978). Behind, it communicates 

 with the Sylvian aqueduct, and through this canal indirectly with the fourth ventricle; 

 anteriorly it connects with the two lateral ventricles by means of the foramina of 

 Monro. Its sagittal diameter, measured between the anterior commissure and the 

 base of the pineal body, is approximately 2.5 cm. The lateral wall of the ventricle 

 (Fig. 976) is formed chiefly by that part of the thalamus which lies below the level 

 of the taenia thalami. On this surface, slightly in advance of the middle, is seen the 

 small oval field of the middle commissure, and in front of this the downward curving 

 elevation produced by the anterior pillar of the fornix. Between the latter and the 

 prominent anterior tubercle of the thalamus lies the foramen of Monro (foramen 

 interventriculare), v/hich establishes communication between the third and the cor- 



Middle commissure 

 / 

 Pineal recess 



,- Foramen of Monro 



Anterior commissure 



Suprapineal recess 



Posterior commissure 

 Sylvian aqueduct 



Mammillary body^ ''^ ^^^''^"^ Optic recess 



Infundibulum  



Optic chiasm 



% 

 Cast of third ventricle, viewed from the side. X 3- {Retzius.) 



responding lateral ventricle, and transmits the trunk formed by the union of the 

 vein of the corpus striatum and the choroid vein. A shallow furrow on the ventric- 

 ular wall, the sulcus hypothalamicus leads from the foramen backward and some- 

 what downward (Fig. 976). It is of importance as indicating, even in the adult 

 brain, the demarcation between the thalamencephalon and the hypothalamus — parts 

 derived respectively from the dorsal and ventral zones of the embryonic brain-vesicle. 

 The roof of the ventricle extends from the foramina of Monro, bounded above 

 and in front by the arching pillars of the fornix, to the pineal body behind, over 

 which it pouches out into the suprapi?ieal recess, as the little diverticulum overlying 

 the body is termed. The immediate and morphological roof consists of the dehcate 

 cpendymal layer, which is attached to the taenia thalami on each side and, stretching 

 across the interthalamic cleft, closes in the ventricle. The ependymal layer, how- 

 ever, is backed by a vascular fold of pia mater, which, in conjunction with the 

 epithelial layer, constitutes the velum interpositiim. This structure is more fully 

 described in connection with the lateral ventricles (page 11 62); but its relation to 

 the third ventricle finds appropriate mention at this place. As in the roof of the 

 fourth ventricle and in the lateral ventricles, so in the third does the vascular tissue 

 of the pia mater invaginate the ependymal layer to form vascular fringes which 

 project into the ventricle (Fig. 974). A double line of such invaginations hangs 

 from the roof of the third ventricle and constitutes the choroid plexus of that space. 

 Since the ependyma everywhere covers these pial processes, it is evident that the 

 fringes are, strictly regarded, outside the ventricle and excluded by the continuous 

 layer of the epithelium. 



