344 NERVOUS SYSTEM. 
In the following description the parts of the brain will be 
taken up in this order: 
A. RHOMBENCEPHALON (primitive hindbrain). 
1. Myelencephaton. 
2. Metencephalon. 
B. MESENCEPHALON (primitive midbrain). 
3. Mesencephalon. 
C. PROSENCEPHALON (primitive forebrain). 
4. Duiencephalon. 
5. Lelencephaton. 
A. RHOMBENCEPHALON.—1. Myelencephalon.—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, 2) ventrally and laterally; by the cerebellum (Fig. 
137, L//) dorsally. The cranial portion of its dorsal surface 
is overhung by the cerebellum. The points of origin of the 
roots of the first pair of cervical nerves (Fig. 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, 4). 
The cavity thus formed is flattened and triangular, with the 
apex of the triangle caudad, and is the caudal part of the 
fourth ventricle (Fig. 141, %). The roof is very thin (Fig. 
143, 2) 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, 7). 
The ventral (anterior) fissure (Fig. 138, ~) of the cord 
passes onto the medulla oblongata and ends at the pons (7), 
its end being slightly deeper and forming what is known as the 
