THE BRAIN. 



379 



swellings — the three primary vesicles. Out of these three 

 embryonic vesicles, the^z/^ regions seen in the adult brain 

 are formed. The median vesicle corresponds to the mid- 

 brain of the adult, \<rhile the anterior vesicle forms the first 

 two, and the posterior vesicle the last two divisions. 



The first vesicle persists in the adult in the region (2) of 

 the optic thalami, or thalamencephalon, or tween-brain. 

 But anteriorly from the first vesicle there arises secondarily 

 the important region (i) of the cerebral hemispheres, or 

 prosencephalon, or fore-brain. 



The median vesicle persists in the adult in the region (3) 

 of the optic lobes, or mesencephalon, or mid-brain. 



The third vesicle persists in the adult in the region (5) of 

 the medulla oblongata, or myelencephalon, or after-brain. 

 But anteriorly from this third vesicle there arises the region 

 (4) of the cerebellum, or metencephalon, or hind-brain. 



Great importance must be attached to the cerebral hemi- 

 spheres, prosencephalon, or fore-brain, for this region pre- 

 dominates more and more in the ascent of Vertebrates, and 

 becomes more and more the home of thought. Between the 

 longitudinal halves into which it is divided, there are several 

 bridges or commissures. With the fore-brain olfactory lobes 

 are also associated. The cavities of the two cerebral hemi- 

 spheres are called the lateral ventricles. 



With the second region — that of the optic thalami, or 

 thalamencephalon, or tween-brain — the following important 

 structures are associated : — (a) The optic outgrowths which 

 form the optic nerves, and some of the most essential parts 

 of the eyes ; (b) the pineal outgrowth or epiphysis which 

 ascends dorsally, and is interpreted with much likelihood as 

 a rudimentary median eye ; {c) the pituitary outgrowth or 

 infundibulum which descends ventrally towards the mouth, 

 and forms part of the enigmatical pituitary body or hypo- 

 physis. The cavity of the thalamencephalon is called the 

 third ventricle. 



The third region — that of the optic lobes, or mesen- 

 cephalon, or mid-brain — has for its floor what are called the 

 crura cerebri, while dorsally lie the two optic lobes, which are 

 hollow in almost all Vertebrates. Through this mid-brain 

 runs a canal or iter or aqueduct of Sylvius connecting the 

 third ventricle with the fourth. 



