534 CHORD AT A. 



bellum and medulla are related to one another as roof and floor of 

 one and the same cavity (fig. 569). The distinction between the 

 first and second vesicles is problematical. The fore brain becomes 

 divided into three parts by an inpushing at its anterior end: an 

 impaired middle portion, and in front a right and a left diver- 

 ticulum. These paired portions, increasing in size, form the cere- 

 bral hemispheres, and together with a small connecting part 

 represent the first cerebral vesicle, while the unpaired portion 

 forms a second vesicle, the 'twixt brain. 



Introducing the terms of human anatomy for the separate parts 

 of the brain, the first vesicle consists of the two cerebral hemi- 

 spheres whose dorsal and lateral walls are usually thick and are 

 called the pallium, while in the floor of each hemisphere is an 

 enlargement, the corpus striatum (cs). The spaces in the hemi- 

 spheres are the first and second ventricles (sv). From the front 

 portion of each hemisphere arises a distinct region, the olfactory 

 lobe (o/), which gives origin to the olfactory nerve. Since the 

 organ of smell is frequently at some distance from the brain, the 

 olfactory nerve must be elongate, as in the Amphibia (fig. 614), or 

 the olfactory lobe must lengthen, as in many Elasmobranchs (fig, 

 592). In the latter case the swollen end of the lobe is close to 

 the olfactory epithelium and is connected with the brain by a long 

 stalk, the tractus, while the swelling is called the bulbus olfacto- 

 rius. Both, as parts of the brain, must be distinguished from the 

 olfactory nerve. 



In the region of the second vesicle only the lateral walls become 

 thickened, producing the optic thalami, directly adjoining the 

 corpora striata; the roof of this vesicle develops no nervous sub- 

 stance, but remains a thin layer of epithelium closing in the third 

 ventricle above (///) The floor is also thin-walled between the 

 thalami and is pushed downwards, forming a funnel-like pocket, 

 the infundibulum (i). The third vesicle, as a rule, is divided by 

 a deep longitudinal dorsal groove, dividing the cavity into a right 

 and left ventricle, while the two halves of the roof are known as 

 the optic lobes or corpora bigemini. In the mammals alone (in 

 ^vhich there is also a transverse groove dividing the optic lobes 

 into the corpora quadrigemini) the cavity of this mid brain is re- 

 duced, by thickening of the walls, to a narrow canal, the iter or 

 aqueduct of Sylvius, with the result that the term fourth ventricle 

 is transferred to the cavity of the hind brain. 



This last region is called the medulla oblongata ; it is a prolonga- 

 tion of the spinal cord, and in many respects shows a similar struc- 



