THE NERVOUS SYSTEM '337 



THE ENCEPHALON. 



The encephalon is the part of the cerebro-spinal axis which is 

 contained within the cranial cavity. It is composed of the medulla 

 oblongata, pons Varolii, cerebellum, and cerebrum. Originally it 

 consists of three hoUow vesicles, namely, the anterior cerebral vesi- 

 cle, which represents the prosencephalon or fore-brain ; the middle 

 cerebral vesicle, representing the mesencephalon or mid-brain ; and 

 the posterior cerebral vesicle, corresponding to the rhombencephalon 

 or hind-brain. 



{Prosencephalon or Fore-brain. 

 Mesencephalon or Mid-brain. 

 Rhombencephalon or Hind-brain. 



The subdivisions of the prosencephalon are the telencephalon 

 and the thalamencephalon or diencephalon ; the mesencephalon 

 remains undivided ; and the subdivisions of the rhombencephalon 

 are the metencephalon and the myelencephalon. 



Prosencephalon = | Thalamencephalon or Diencephalon. 

 Mesencephalon = Mesencephalon. 

 Rh.mbe„oephal«n= { SjSSp^ri. 



The various parts of the encephalon which are developed from 

 these subdivisions will be made evident from the following table : 



i Cerebral Hemispheres. 

 Lateral Ventricles. 

 iviuuuepuaiuu = • Anterior peirt of Third Ventricle. 



Foramina of Monro. 

 \^ Olfactory Lobes. 



Posterior Part of Third Ventricle. 



Thalamencephalon ] | pP^^'^g^^y™' ^"'^ ^""^'^ Geniculata. 



r»:„«-«li,„i-«., r I Interpeduncular Structures. 

 Diencephalon J Pituitary Body. 



I Optic Nerve and Retina. 



j Corpora Quadrigemina. 

 Mesencephalon = - Crura cerebri. 



I Aqueduct of Sylvius. 



f Cerebellum. 

 Metencephalon = < Pons Varolii. 



( Pontine Part of the Foarth Ventricle. 



Mvelencenhalon - ^ Medulla Oblongata or Bulb, 



myeiencepnaion - ^ -q^^^^j. p^rt of Fourth Ventricle. 



General Description of the Base and Superior Surface of the 

 Encephalon. 



The inferior aspect of the encephalon is kno\vn as the base. In the 

 following general description of the parts which it presents the order 

 pursued is, as nearly as possible, from behind forwards and upwards. 



