660 



THE NERVOUS SYSTEM. 



losum forms the roof of the lateral ventricles, while in the mesial plane it is con- 

 tinuous behind with the fornix, being separated from it in front by the septum 

 lucidum, which forms a vertical partition between the two ventricles. On each side 

 the fibres of the corpus callosum extend into the substance of the hemispheres, con- 

 nectino- them together. The greater thickness of the two extremities of this com- 

 missure is explained by the fact that the fibres from the anterior and posterior parts 

 of each hemisphere cannot pass directly across, but have to take a curved direction. 



FIG. 353. Vertical median section of the encephalon, showing the parts in the middle line. 



1. Convolution of the corpus callosum. Above it 



is the calloso-marginal fissure. 



2. Fissure of Rolando. 



8. The parieto-occipital fissure. 



4. 4, point to the calcarine fissure, which is just 



above the numbers. Between 2 and 3 are the 

 convolutions of the quadrate lobe. Between 3 

 and 4 is the cuneate lobe. 



5. The corpus callosum. 



6. The septum lucidum. 



7. The fornix. 



8. Anterior crus of the fornix, descending to the 



base of the brain, and turning on itself to form 

 the corpus albicans. The bundle of Vicq 

 d'Azyr is indicated by a dotted line. 



9. The optic thalamus. Behind the anterior crus 



of the fornix a shaded part indicates the fora- 

 men of Monro; in front of the number an 

 oval mark shows the position of the gray or 

 middle commissure. 



10. The velum interpositum. 



11. The pineal gland. 



12. The corpora quadrigemina. 



13. The cruscerebri. 



14. The valve of Vieussens (to the right of the 



number). 



15. The pons Varolii. 

 l(i. The third nerve. 



17. The pituitary body. 



18. The optic nerve. 



19 points to the anterior commissure, indicated by 

 the oval outline behind the number. 



The part of the corpus callosum which curves forward on each side from the germ 

 into the frontal lobe and covers the front part of the anterior cornu of the lateral 

 ventricle is called the forceps anterior or minor. The part which curves backward 

 from each side of the splenium into the occipital lobe is known as the forceps pos- 

 terior or major. Between these two parts on each side is the main body of the 

 fibres, which extend laterally into the temporal lobe and cover in the body of the 

 lateral ventricle. These are known as the tapetum or mat. 



An incision should now be made through the corpus callosum, on either side of the raphe, 

 when two large irregular cavities will be exposed, which extend through a great part of the 

 length of each hemisphere. These are the lateral ventricles. 



The Lateral Ventricles (Fig. 354). The lateral ventricles, two in number, right 

 and left, are irregular cavities situated in the lower and inner parts of the cerebral 



