CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 261 



On ^ removing the upper part of the skull (with the ossified 

 tentorium and the unossified part of the dura mater) the surface of 

 the brain comes into view as an ovoid, convex mass, consisting of 

 two large anterior portions, with a few large contorted prominences 

 on their surface, and a smaller pos- 

 terior part marked with numerous 

 small transverse folds or furrows. 

 The two large anterior portions 

 are the cerebral hemispheres, the 

 median, posterior part is the cere- 

 bellum the anterior part of the 

 upper surface of which is overlapped 

 by the hinder portions of the cere- 

 bral hemispheres. The anterior ends 

 of the two olfactory lobes are also 

 to be seen projecting in front of the 

 middle of the anterior end of the 

 cerebrum (Fig. 126, ol). The median 

 line which divides the cerebrum 

 into its two lateral halves, or hemi- 

 spheres, is called the great or 

 median longitudinal fissure. The 

 smooth, contorted prominences on 

 the surface of the cerebrum, are 

 called the convolutions or gyri t the 

 depressions which separate them 

 are termed sulci, or fissures. These 

 gyri and sulci are distinguished by 

 definite names ; but it will be better 

 to defer their description till a further 

 acquaintance has been made with 

 the brain as a whole, and with all 

 its main constituent portions. The 

 pia mater so closely invests the brain 



that it passes down not only into the great longitudinal fissure, but also 

 into all the sulci of the cerebrum and into the numerous folds on 

 the surface of the cerebellum. The dura mater passes into the 

 great longitudinal fissure (the membranous fold dipping into it being, 

 as before said, known as the falx), and between the cerebrum and 

 cerebellum (the structure known as the tentorium), but it does not 

 descend into the minor depressions of the brain surface. 



If the two cerebral hemispheres be pushed apart, a large trans- 

 verse white band of fibres, called the corpus callosum, will come into 

 view, which band connects the two hemispheres for rather more 

 than the middle third of their antero-posterior extent (Fig. 129, cc). 



If the hinder ends of the cerebral hemispheres be forcibly divari- 

 cated, then the layer of brain substance, before spoken of as extending 

 forwards from the front of the cerebellum, will come into view. 

 Upon its surface, immediately in front of the cerebellum, two pairs 



Fig. 126. UPPER SURFACE OF BRAIN, 

 SHOWING THE DEEP LONGITUDINAL, FlS- 

 SURE DIVIDING THE TWO HEMISPHERES 

 AND THE CEREBELLUM BEHIND THEM. 



c. Crucial sulcus. 



s. Superior external gyrus. 



m. Middle -external gyrus. 



i. Inferior external gyrus. 



v. Vermiform process of cerebellum. 



ol. Olfactory lobe. 



