IX SPINAL CORD, BRAIN 329 



The brain of the vertebrate is for descriptive purposes divided into 



the undermentioned regions, but it must be distinctly understood that 



the brain is a fundamentally continuous whole and that the division into 



the portions named is purely secondary, having come about with the 



gradual specialization of particular parts of the brain for particular 



functions. 



[ Hemispheres. 



Cerebrum \ Thalamencephalon. 



I Mesencephalon. 



C Cerebellum. 

 Rhombencephalon -', ,, 



I. Medulla oblongata. 



The Medulla oblongata (Fig. 138, A, m.o) is a direct forward 

 prolongation of the spinal cord, from which it differs in three well-marked 

 features': (i) Its diameter is greater, (2) the portion of central canal 

 within it is greatly expanded forming the Fourth Ventricle, (3) its roof 

 is degenerate, being a thin membrane devoid of ganglion-cells and 

 nerve-fibres and having closely apposed to its outer surface a rich net- 

 work of blood-vessels — a so-called choroid plexus. Physiologically the 

 great importance of the medulla oblongata lies in the fact that in its 

 side-walls and floor are situated the masses of ganglion-cells which form 

 the special nerve-centres (" nuclei ") belonging to those nerves of the 

 head region which are of the greatest importance to life. 



The Cerebellum (Fig. 138, A, c) is simply the front portion of the 

 roof of the fourth ventricle which, instead of degenerating into a mem- 

 branous condition as does the rest, forms a thick transverse band stretch- 

 ing across from side to side. As development goes on this transverse 

 band undergoes very active growth in an antero-posterior direction, with 

 the result that it bulges outwards and forms in the adult a very large 

 and conspicuous portion of the brain, overlapping the brain regions 

 immediately in front and immediately behind it. Functionally the 

 cerebellum appears to be concerned especially with the co-ordination of 

 the contraction of the various muscles of the body, i.e. with securing 

 that the various muscles contract in such a way as to work together for 

 the production of appropriate movement of the body as a whole. 



Of the Mesencephalon (" mid-brain ") the most conspicuous portions 

 are two rounded bulgings of its roof (Fig. 138, A, op.l) — the optic lobes, 

 partially hidden in dorsal view by the cerebellum — ^which contain ganglion- 

 cells concerned with the sense of sight. 



The Thalamencephalon (Fig. 138, A, th) possesses this feature in 

 common with the medulla oblongata that its roof is, at least in parts, 



