THE NERVOUS SYSTEM i345 



visible on, the base of the brain, but behind the corpus callosum it 

 only extends to the level of the tentorium cerebelli, which separates 

 the cerebeUiim from the posterior parts of the cerebral hemispheres. 

 The fissure, therefore, in this situation is not visible inferiorly until 

 the cerebellum and the tentorirun cerebelli have been removed. 



Each hemisphere is semi-ovoid, its mesial or inner surface being 

 flat. The anterior and posterior extremities are rounded, the 

 former being the thicker of the two. The anterior extremity is 

 known as the frontal pole, and the posterior extremity forms the 

 occipital pole. The surface of each hemisphere consists of grey 

 matter, which is spoken of as the cerebral cortex. Superiorly and 

 externally it is convex in adaptation to the concavity of the vault 

 of the cranium. It is broken up into a number of tortuous emi- 

 nences, caUed gyri or convolutions, and these are separated from 

 each other by clefts, called sulci or fissures. The surfaces of the 

 gyri which bound the sulci are covered with grey matter, like their 

 exterior. The pia mater closely covers the gyri, and also dips into 

 the sulci, so as to cover the opposed surfaces of the gyri. The 

 arachnoid membrane, however, does not dip into the sulci, but passes 

 over them. The sulci are of various dfepths, but the average depth 

 may be said to be \ inch. 



RHOMBENCEPHALON. 



I. The Medulla Oblongata. 



The medulla oblongata or bulb is continuous with the spinal cord 

 and extends from the lower margin of the foramen magnum of the 

 occipital bone to the lower border of the pons Varolii. Its direction 

 is upwards and slightly forwards, and it measures i inch in length, 

 I inch in breadth at the widest part, and fully ^ inch in thickness. 

 Inferiorly its girth corresponds with that of the spinal cord, but it 

 widens superiorly, so that it is somewhat pyramidal. Its ventral 

 surface faces the basilar groove of the occipital bone, and its dorsal 

 surface is directed towards the vallecula of the cerebelliun. 



The bulb is composed of two symmetrical halves, its bilateral 

 symmetry bemg indicated superficially by upward prolongations 

 of the ventral or anterior and dorsal or posterior median fissures of 

 the spinal cord. The anterior median fissure extends as high as the 

 lower border of the pons Varolii, where it expands slightly and 

 forms a blind recess, called the foramen ccBCum. In its lower part 

 this fissure is interrupted and crossed by bundles ot nerve-fibres, 

 which are derived from the inner three-fourths of each p\Tamid, 

 the decussation thus formed being known as the decussation of the 

 pyramids, or motor decussation. The posterior median septum only 

 extends along the lower half of the bulb, and it terminates superiorly 

 at the level of the calamus scriptorius. 



85 



