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MANUAL OF PHYSIOLOGY. 



The cavity of the thalamencephalon opens behind into the 

 cavity of the middle cerebral hemisphere, and in front it commu- 

 nicates with the hollow rudiments of the cerebral hemispheres, 

 and eventually it becomes the cavity of the third ventricle. The 

 floor of the thalamencephalon is ultimately developed into the 

 optic chiasma and part of the optic nerves, as well as the infun- 

 dibulum. The latter comes in contact with a process from the 

 mouth, which is ultimately changed into the pituitary body. The 

 anterior part of the roof of the thalamencephalon becomes very 

 thin, and its place is finally occupied by a vascular plexus, which 



V-XH 



Diagram of a vertical longitudinal section of the developing brain of a vertebrate ani- 

 mal, showing the relation of the three cerebral vesicles to the different parts of the adult 

 brain. (Huxley.) Olf. Olfactory lobes. F.M. Foramen of Monro. C.S. Corpus stri- 

 atum. Th. Optic thalamus. Pn. Pineal gland. M.b. Mid-brain. Cb. Cerebellum. M.O. 

 Medulla oblongata. Hmp. Central hemispheres. ThE. Thalamencephalon. Py. Pitui- 

 tary body. C.Q. Corpora quad rigemina. C.C. Crura cerebri. P. V. Pons Varolii. /.- 

 XII. Regions from which spring the cranial nerves. 1. Olfactory ventricle. 2. Lateral 

 ventricle. 3. Hind ventricle. 4. Fourth ventricle. 



persists in the roof of the third ventricle (choroid plexus). The 

 pineal gland a peculiar outgrowth of unknown function is 

 developed from the posterior part of the roof of the thalamen- 

 cephalon, and from its sides, which become extremely thickened, 

 are developed the optic thalami. 



The primitive cerebral hemispheres first appear as two lobes 

 growing out from the front of the anterior part of the first cerebral 

 vesicle. The floor of these lobes thickens to give rise to the cor- 

 pora striata, and the roof develops into the hemispheres proper. 

 The cavities of these lobes become the lateral ventricles, and are 



