THE BRAIN. 83 



the skull. These cavities are merely 'parts of, or outgrowths of, the 

 original central canal of the neural tube of the embryo. {Gf. 

 Chap. IX.) 



i. The lateral ventricles extend through the whole 

 length of the cerebral hemispheres and a short 

 way into the olfactory lobes. 



ii. The third ventricle is situated in the thalamen- 

 ^ cephalon : it opens in front through the foramina 



of Monro into the lateral ventricles : the stalk 

 of the pineal body opens into it above ; and in 

 the hinder part of its floor is a conical depression, 

 the infundibulum. 



iii. The aquaeductus Sylvii or iter a tertio a quartum 

 ventriculum is a narrow passage leading from the 

 third to the fourth ventricle : it communicates 

 above with the cavities or ventricles of the optic 

 lobes, which are hollow. 



iv. The fourth ventricle is the large triangular cavity 

 in the medulla, already exposed by removal of the 

 vascular membrane coverino' it. 



O 



c. The ventral surface of the brain. 



Cut through the medulla at the level of the hinder end of the 

 skidl : carefully remove the brain from the cranial cavity, noting 

 the several nerves arising from it, and cutting through these as far 

 from the brain as possible. Lay the brain on its dorsal surface, 

 and examine and draw the ventral surface, showing the following 

 parts. 



i. The optic cMasma is formed by the decussation 

 of the roots of the optic nerves ; the point of 

 crossing being opposite the hinder ends of the 

 hemispheres, and immediately in front of the 

 infundibulum. 



Trace back the optic nerves behind their point of crossing to 

 their origins from the ojJtic lobes. 



ii. The tuber cinereum is a small median swelling 

 immediately behind the optic chiasma, caused by 

 the depression of the floor of the third ventricle 

 to form the infundibulum. It is divided by a 

 median groove into right and left halves. 



