126 ANATOMY. 



line, between the hippocampus major and the optic thalamus. It opens 

 at the base of the brain between the middle lobe and the crus cerebri, 

 and admits the pia mater to the middle cornu. 



Describe and bound the Third Ventricle. It is a mere fissure, situated 

 between the optic thalami in the median line. It communicates with the 

 lateral ventricles above by the foramina of Monroe, and with the 4th ventricle 

 behind by the iter e tertio ad quartam ventriculum. It is crossed by 3 bands, 

 the anterior, middle, and posterior commissures, the middle one being of gray 

 matter, the others of white. In the foetus its cavity communicates with the 5th 

 ventricle, and through the infundibulum with the cavity of the pituitary body. 

 It is bounded as follows : 



Roof, the velum interpositum, and above it the fornix. 



Floor, the parts comprised in the interpeduncular space at the base of the 

 brain, viz. the lamina cinerea, tuber cinereum, infundibulum, corpora 

 albicantia, and posterior perforated space. 



Anteriorly, the anterior crura of the fornix, and the anterior commissure 

 of the ventricle. 



Posteriorly, the posterior commissure, and the iter e tertio, etc. 



Laterally, the optic thalami and the peduncles of the pineal gland. 



Describe and bound the Fourth Ventricle. The fourth, or ventricle of 

 the cerebellum, lies between the medulla oblongata and the cerebellum. It is 

 considered by some to be a dilatation of the central canal of the spinal cord, 

 and is enclosed posteriorly by the pia mater ; an opening in which admits the 

 entrance and exit of the sub-arachnoid fluid from the sub-arachnoidean space of 

 the brain and spinal cord, and a fold of the pia mater called the Choroid 

 Plexus. It also communicates with the 3d ventricle by the iter e tertio, etc. 

 Its boundaries are as follows : 



Roof, the valve of Vieussens, and the cerebellum. 



Anteriorly, the medulla oblongata, and the pons Varolii. 



Posteriorly, the cerebellum. 



Laterally, the processus e cerebello ad testes, the posterior pyramids and 

 the restiform bodies of the medulla oblongata. 



Floor, the posterior median fissure of the medulla, the orifice of the cen- 

 tral canal of the cord, the locus ceruleus and taenia violacea (both of blue 

 color), the fasculi teretes (2 spindle-shaped elevations) and eminences of 

 origin of certain nerves. 



Describe the Fifth Ventricle. It is situated between the two layers of the 

 septum lucidum, and therefore between the lateral ventricles. Its roof is formed 

 by the corpus callosum, and in the foetus it communicates with the 3d ventricle 

 by an opening between the pillars of the fornix. It usually contains fluid. 



