THE NERVOUS SYSTEM 47 



cephalon. The optic chiasma is formed by the crossing of 

 the optic nerves. Posterior to the optic chiasma the floor 

 of the diencephalon bulges ventrally and posteriorly form- 

 ing the infundibulum made up of two inferior lobes. From 

 between the two inferior lobes a sac, the hypophysis or 

 pituitary body projects caudad. It is a gland of internal 

 secretion in mammals. Parts not identifiable on the brain 

 will often be found adhering to the floor of the brain case. 



5. The Ventricles of the Brain. 



The brain retains the hollow spaces of its early tube con- 

 struction as the ventricles or cavities of the brain. 



The fourth ventricle is the most posterior of these. It is 

 the cavity within the medulla oblongata. Its ventral por- 

 tion is the fossa rhomboidea. Make a ventral sagittal cut 

 through the brain and examine under water. 



The aqueduct of the brain extends anteriorly from the 

 fourth ventricle. It communicates with the cavity, the 

 cerebellar ventricle of the cerebellum and the optic ven- 

 tricles of the optic lobes. The third ventricle is in the 

 diencephalon, the roof of which consists of the vascular 

 choroid plexus which folds into the cavity. 



Just in front of the optic lobe is a small thickened por- 

 tion of the diencephalon, the habenula, from which a slen- 

 der process, the pineal body, extends dorsally. The third 

 ventricle connects by the ventricle-of-Monroe or inter- 

 ventricular foramen with the very small lateral ventricles 

 which represent the first and second ventricles. 



