26 ELEMENTARY BIOLOGY. [CHAP. 



On each side, the cavity of the third ventricle is bounded 

 by a thick mass of nervous matter into which the crura 

 cerebri pass. These are the optic thalami. The roof of 

 the third ventricle is very thin and easily torn through ; its 

 fore part is prolonged up to form a delicate process in con- 

 nexion with an ovate body, which is lodged between the 

 posterior parts of the cerebral hemispheres and represents 

 a portion of the pineal gland. The front part of the floor of 

 the ventricle, on the other hand, is produced into a back- 

 wardly directed process, the infundibulum. This is con- 

 nected below with a highly vascular mass, the pituitary 

 body. 



The hemispheres are elongated bodies, broader behind 

 than in front, where they are marked off only by a slight 

 constriction from the olfactory lobes. 



Anteriorly, the wall of the third ventricle is thickened 

 to form the lamina terminalis ; on each side, between this 

 and the peduncle of the pineal gland, is a small aperture, 

 the foramen of Monro, which places the median third- 

 ventricle in communication with the paired lateral ventricle 

 of the cerebral hemisphere. 



The lateral ventricle is continued on either side into the 

 base of the olfactory lobe to form the olfactory ventricle. In 

 front these lobes become nerve-like cords, which leave 

 the skull and spread out on the posterior faces of the 

 olfactory sacs. 



The inner faces of the olfactory lobes are confluent with 

 each other, and there pass between corresponding parts 

 of opposite halves of the brain bands of fibres or com- 

 missures (see Laboratory work) the most important of which 

 runs through the substance of the lamina terminalis and 

 represents the corpus callosum. 



The entire cerebro-spinal axis is invested in a highly 



