344 NERyOUS SYSTEM. 



In the following description the parts of the brain will be 

 taken up in this order: 



A. Rhombencephalon (primitive hindbrain). 



1 . Myelenceplialon. 



2 . Metenceph a Ion. 



B. Mesencephalon (primitive midbrain). 



3. UlcscnccpJialon. 



C. Prosencephalon (primitive forebrain). 



4. Dicnccplialon. 



5 . TclotccpJialon. 



A. Rhombencephalon. — i. Myclenccphalon. — The my- 

 elencephalon or medulla oblongata is transitional between the 

 spinal cord and the brain. It has in general the structural 

 peculiarities of the cord, but these undergo in it a gradual trans- 

 formation into the arrangements characteristic of the brain. 



It has the form of a flattened and truncated cone, which 

 widens craniad and is limited at the cranial end by the pons 

 (Fig. 138, z) ventrally and laterally; by the cerebellum (Fig. 

 137, ///) dorsally. The cranial portion of its dorsal surface 

 is overhung b)' the ccrcl^elhun. The points of origin of the 

 roots of the fust pair of cervical nerves (I'ig. 138, s) may be 

 taken as indicating the boundary between the spinal cord and 

 the medulla oblongata; there is no other external marking to 

 show the limits of the two. 



The central canal of that part of the medulla which is 

 overhung by the cerebellum is greatly widened (Fig. 141, //). 

 The cavity thus formed is flattened and triangular, with the 

 apex of the triangle caudad, and is the caudal part of the 

 fourth ventricle (l-'ig. 141, h). The roof is very thin (Fig. 

 143, li) and is intimately connected with the pia mater, so that 

 in removing the latter the thin roof of the fourth ventricle is 

 removed with it and the cavity of the fourth ventricle is left 

 exposed. The thin roof of the fourth ventricle is known as the 

 velum medullare posterius (Fig. 143, //). 



The ventral (anterior) fissure {y\Z- 138, /) of the cord 

 passes onto the medulla oblongata and ends at the pons (/), 

 its end being slightly deeper and forming what is known as the 



