292 ESSENTIALS OF ZOOLOGY 



the brain underneath the body. The f orebrain gives off laterally 

 the paired diverticula, which form the cerebral hemispheres 

 and the paired optic cups. The ventricles of the cerebral 

 hemispheres are the lateral ventricles, and communicate with 

 the original forebrain by the foramina of Monro. The vesicle 

 into which they open is the third ventricle, and this region of the 

 brain is called the thalamencephalon. Posteriorly, this ven- 

 tricle passes into the cavity of the mesencephalon, and there 

 it is narrowed to form the iter, or aqueduct of Sylvius. The 

 iter passes in turn into the fourth ventricle, which is the cavity 

 of the hindbrain, and it is continuous with the central canal 

 of the spinal cord. 



The cerebral hemispheres, or prosencephalon, undergo a 

 great expansion in the rabbit, as in all mammals. Their 

 structure may be followed by dissection and by making trans- 

 verse sections, and for the purpose it is advisable to obtain the 

 brain of a larger mammal than the rabbit. The brain of the 

 sheep is conveniently obtained, and will be described. It 

 may be found advantageous to get the head of a sheep and 

 study it in longitudinal section. The hemispheres form large 

 paired masses which extend over the median part of the b/ain, 

 abutting against the cerebellum. The olfactory lobes lie 

 underneath the anterior ends, and the immensely thickened 

 walls are thrown into a series of folds which it will be observed 

 are on the whole paired. Dorsally, the fissura cruciata is 

 marked and divides the frontal from the parietal lobes of the 

 hemisphere. Laterally, the Sylvian fissure separates the 

 frontal and temporal lobes. The surface of each of these 

 lobes is greatly increased in area by the development of 

 grooves called sulci, defining the folds or convolutions called 



On the ventral side the frontal and temporal lobes are 

 limited by the rhinal fissure, internal to which are the olfactory 

 tracts. The most prominent feature of this basal area of the 

 hemisphere is the pyriform area, which extends from the 

 olfactory lobe to the hinder part of the floor. 



A median section separating the two hemispheres will show 

 that the separation of the hemispheres is not complete. The 

 two ventricles come close together, and are separated by the 



