NEUROLOGY. 113 



siderable depth, called the ßssure of Sylvius^ is the posterior limit of each 

 anterior segment. The fissure of Sylvius corresponds on each side to the 

 posterior concave edge of the ala of the sphenoid bone, which is received 

 within it. Commencing from within, at a triangular flat surface {locus per- 

 foratus anticm\ at the posterior extremity of each olflictory process, it pro- 

 ceeds outwards and curves backwards, its convexity directed forwards to 

 the lateral surface of the brain. It is of considerable depth, especially at its 

 internal extremity, and like all fissures of the brain, is lined by pia mater. 

 The island of Reil is found at the bottom of this fissure, projecting from its 

 floor as a small insulated lobe, bounded by a bifurcation of the fissure. 



The middle segment^ which lies immediately behind the fissure of Sylvius, 

 is on a plane much lower than the anterior, and corresponds on each side 

 to the deep and hollow median fossa of the cranium. It consists of two 

 lateral, very convex lobes, known as the middle lobes of the brain, which are 

 separated from each other by a deep depression. These lobes, which are 

 distinctly defined in front by the fissure, have no exact boundary behind, 

 but pass off very gradually into the posterior lobes of the hemispheres, as may 

 be seen by raising up the cerebellum. 



Commencing at the anterior fissure and passing backwards we shall per- 

 ceive the following parts. This fissure is limited by the anterior fold or 

 reflexion of the corpus callosum ; behind this is a thin layer of a greyish 

 color which principally constitutes the floor of the third ventricle, and is 

 known as the tuber cinereum. The pituitarg process is continuous with it, 

 being probably its extension, and is a hollow process of nervous matter 

 above the sella turcica by which the brain is in a measure tied to the pitui- 

 tary body. 



Immediately in front of the pituitary process, the union of two white 

 bands, which form lateral boundaries to a large portion of the tuber cine- 

 reum, the optic tracts, takes place along the median line. This forms the 

 commissure of the optic nerves from which these nerves diverge. Behind 

 the pituitary process, the tuber cinereum extends back to two small and 

 very white bodies called corpora raammillaria or albicantia. 



Behind the mammillary bodies, we find a deep depression, lying between 

 two thick processes of fibrous matter, which pass from below upwards and 

 outwards, expanding as they advance, and upon which each hemisphere is 

 sustained like a mushroom on its stalk. These are the crura cerebri, the 

 peduncles of the cerebral hemispheres. The depression separating them is 

 the iritercrural space. The nervous matter forming the floor of this space 

 is of a greyish color and known as the pons Tarini. The third pair of 

 nerves emerges from the interpeduncular space. 



The inner margin of each middle lobe of the brain is separated from the 

 corresponding crus cerebri by a fissure which passes from behind forwards, 

 and terminates in the fissure of Sylvius. Backwards this fissure is continu- 

 ous with the transverse fissure separating the cerebnmi from the cerebellum, 

 and corresponding to the posterior edge of the corpus callosum. The con- 

 tinuity thus established between these three series of fissures constitutes the 

 great cerebral or transverse fissure. 



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