672 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



is lined by the pia mater. We notice here a 

 large interval between the arachnoid and pia 

 mater, in which a considerable accumulation 

 of the cerebro-spinal fluid takes place, com- 

 municating with the anterior conflux of that 

 fluid. In this space runs the middle artery of 

 the brain, giving off its branches to the sides 

 and floor of the fissure. When the convolu- 

 tions which bound the fissure are separated, 

 a variable number of small convolutions is 

 found, projected from its floor as an insulated 

 lobe, which is enclosed by a bifurcation of the 

 fissure. This lobe constitutes the island (insel) 

 of Reil. 



The middle segment which lies immediately 

 behind the Sylvian fissure, is on a plane much 

 lower than the anterior, and corresponds on 

 either side to the deep and hollow median 



fossa of the cranium. It consists of two la- 

 teral very convex lobes, commonly known 

 as the middle lobes of the brain, which are 

 separated from each other by a deep depres- 

 sion. These lobes, which are very accurately 

 limited 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. 



The transition from the middle to the pos- 

 terior lobe of the hemisphere is only indicated 

 by the different character of the inferior surface 

 of the hemisphere, the former being convex, 

 the latter concave. The subdivision, indeed, 

 of the cerebral hemisphere into middle and 

 posterior lobes is purely conventional, and I 

 agree with Cruveilhier that it ought to be dis- 

 carded, for it has no foundation in the anatomy 



Fig. 382. 



Base of encephalon viewed from below. 



A, anterior lobe; B, middle lobe; C, posterior lobe; D, cerebellum; a, olfactory nerves; 6, optic 

 nerves; e, third pair of nerves; d, fourth pair of ditto; e, fifth pair portio major; e', fifth pair 

 portio minor ; f, sixth pair ; g, seventh pair ; h, filaments of origin of the glosso-pharyngeal and 

 vagus; i, spinal accessory nerve; k, ninth nerve ; /, pituitary body and process proceeding from the 

 tuber ciuereum ; m, uuinnllary bodies ; n, pons Varolii ; o, medulla oblongata. 



