622 STRUCTURE OF THE BULB, PONS, & MID-BRAIN. [CH. 



the body, though later in life this is in a sense re-established by 

 the nerves that grow from the brain and cord to the surface. 

 The anterior end of this tube becomes greatly thickened, to form 

 the brain, its cavity becoming the cerebral ventricles ; the rest 

 of the tube becomes the spinal cord. The primitive brain is at 

 first subdivided into three parts, the primary cerebral vesicles; 

 the first and third of these again subdivide, so that there are 

 ultimately five divisions, which have received the following 

 names : 



1. Pros-encephalon, or fore brain. This is developed into 

 the cerebrum with the corpora striata. It encloses the lateral 

 ventricles. 



2. Thalam-encephalon, or twixt brain. This is developed 

 into the parts including the optic thalami, which enclose the 

 third ventricle. 



3. Mes-encephalon, or mid brain, consists of the parts which 

 enclose the aqueduct of Sylvius namely, the corpora quadri- 

 gemina, which form its dorsal, and the crura cerebri, which form 

 its ventral aspect. 



4. Ep-encephalon, or hind brain, which forms the cere- 

 bellum and pons. 



5. Met-encephalon, or after brain, which forms the bulb or 

 medulla oblongata. 



Figs. 460 and 461 represent a diagrammatic view of a verte- 

 brate brain ; the attachment of the pineal gland, pituitary body, 

 and olfactory (I) and optic (II) nerves is also shown. 



CHAPTER XLIV. 



STRUCTURE OF THE BULB, PONS, AND MID-BRAIN. 



WE may study the bulb and pons by examining first the 

 anterior or ventral, then the posterior or dorsal aspect, and last 

 of all the interior. 



Anterior Aspect. 



The bulb is seen to be roughly shaped, like an inverted trun- 

 cated cone, larger than the spinal cord, and enlarging as it goes 

 up until it terminates in the still larger pons (fig. 462, p). In the 



