THE CEREBRUM 



THE BRAIN 



The Parts of the Central Nervous System. The Brain is 

 divided into two symmetrical hemispheres by the great longi- 

 tudinal fissure, but the two halves are connected to one 

 another by commissural bands, of which the corpus callosum 

 (p. 10) is the most important. The mid-brain descends from 

 the middle of the ba?al surface of the brain and is continuous 

 below with the pons, which in turn becomes continuous with 

 the medulla oblongata. These three structures form the 

 brain-stem and, together with the cerebellum, which projects 

 backwards behind them, they occupy the posterior cranial 

 fossa. At the foramen magnum in the occipital bone the 

 medulla oblongata becomes continuous with the spinal medulla 

 (spinal cord). 



The Lateral Surface of the Brain 



The Central Sulcus (of Rolando) is the most important 

 sulcus on the lateral surface of the cerebral hemisphere. It 

 is directed obliquely downwards and forwards and is situated 

 between two parallel and nearly vertical convolutions, which 

 are termed the anterior and the posterior central gyri. In- 

 feriorly, the central sulcus terminates a little above the 

 posterior ramus of the lateral fissure (of Sylvius) (Fig. 3). 



The grey matter of the anterior central gyrus and of the 

 anterior wall of the central sulcus contains the higher motor 

 centres. The centre for the muscles of the lower limb is situ- 

 ated in the uppermost part of the anterior central gyrus, and 

 it extends over the supero-medial border of the hemisphere 

 for a short distance on to the medial surface (Fig. 6). 

 Immediately below the centre for the lower limb, and 

 slightly overlapping it, lies the centre for the muscles of the 

 trunk, while the upper limb centre is placed a little lower and 

 occupies that part of the anterior central gyrus which pro- 

 jects backwards following the curve of the central sulcus 



