THE NERVOUS SYSTEM. 103 



c. The white substance is in the centre. Is it cellular or 

 fibrous ? How is it arranged ? 



d. The corpus callosum, a flat, thick band of white tissue 

 curved under at either end and situated in the lower part 

 of the cerebral hemispheres. 



e. The cavities of the brain, four of 

 which can be easily distinguished. 



1. The lateral ventricles are in the 



right and left cerebral hemi- 

 spheres. They will be found 

 after cutting the horizontal 

 sections (Fig. 10, I and II). 



2. The third ventricle is dorsal to the \ " "/ " 



optic tracts. (Fig. 10, III.) 



3. The foramen of Monroe is the 



passage from the lateral to 

 the third. It is I^shaped. 



FIG. 10. A diagram 

 (Fig. 10, F.M.) of the ventricles 



4. The fourth ventricle is in the me- ofthebraiu - 



dulla oblongata, and is merely covered by the 

 membranes of the spinal cord. (Fig. 10, IV.} 



5. The aqueduct of Sylvius leads from the third to 



the fourth ventricle. (Fig. 10, A.S.) 

 /. Sketch. 



II. A HORIZONTAL SECTION. 



Cut away the dorsal surface of one hemisphere in thick 

 horizontal sections nearly down to the corpus callosum, 

 noting the arrangement of white and gray matter. Make 

 a shallow, vertical cut into the corpus callosum, parallel 

 with the median fissure and about half an inch from it. 



