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HANDBOOK OF PHYSIOLOGY. 



thalamus. The posterior pillars pass down and out and form part of 

 the interior of that part of the lateral ventricle which descends into the 

 posterior lobe of the cerebrum. Thus, when the fornix is reflected from 

 the front, first of all the velum interpositum is seen, and when that is 

 removed the third ventricle comes into sight. 



The third ventricle terminates at its posterior extremity in the pineal 

 body. From this ventricle a short narrow passage, the iter a tertio ad 



Fig. 355. Dissection of brain, from above, exposing the lateral fourth .and fifth ventricles 

 with the surrounding parts. ^. a, Anterior part, or genu of corpus callosum; 6, corpus stria- 

 tum; 6', the corpus striatum of leftside, dissected so as to expose its gray substance; e, points 

 by a line to the teenia semicircularis ; d, optic thalamus; e, anterior pillars of fornix divided; 

 below they are seen descending in front of the third ventricle, and between them is seen part of 

 the anterior commissure; in front of the letter e is seen the slit-like fifth ventricle, between the 

 two laminee of the septum lucidum ; /, soft or middle commissure ; g is placed in the posterior 

 part of the third ventricle ; immediately behind the latter are the posterior commissure (just 

 visible) and the pineal gland, the two crura of which extend forward along the inner and up- 

 per margins of the optic thalami; h and /, the corpora quadrigemina ; fc, superior crus of cere- 

 bellum; close to fc is the valve of Vieussens, which nas /been divided so as to expose the fourth 

 ventricle; I, hippocampus major and corpus fimbriatum, or taenia hippocampi; m, hippocampus 

 minor; n, eminentia collateralis; o, fourth ventricle; p, posterior surface of medulla oblongata; 

 r, section of cerebellum; s, upper part of left hemisphere of cerebellum exposed by the removal 

 of part of the posterior cerebral lobe. (Hirschfield and Leveille. ) 



quartum ventriculum, or aqueduct of Sylvius, passes through the next 

 portion of the brain called the mid-brain. This part is covered in by 

 two pairs of nerve-ganglia, the anterior and the posterior corpora qua- 

 drigemina, and the floor is formed by the crura cerebri. The aqueduct 

 of Sylvius opens at the upper angle of a lozenge-shaped cavity, the 

 fourth ventricle, which is situated on the dorsal aspect of the pons and 

 bulb. The fourth ventricle has no roof of its own beyond a layer of 



