CHAP, ix.] NERVOUS SYSTEM AND OEGANS OF SENSE. 265 



connecting the lobes with the under surface of the cerebrum are the 

 crura, or peduncles of the olfactory lobes. 



Upon turning back the optic tracts at their union in the optic 

 commissure a delicate layer is seen to connect them with the 

 anterior end of the corpus callosum. This delicate layer is called 

 the lamina cinerea, or lamina terminalis. It is also continuous, 

 below the optic commissure, with the tuber cinereum, and it is con- 

 nected on each side with the locus perforates anterior. 



As has been said, the minute cavity of the spinal cord expands 

 within the brain into a series of chambers, filled with fluid, termed 

 " ventricles." 



The hindmost o? fourth ventricle is placed between the cerebellum 

 and the medulla oblongata. It is a flattened, somewhat rhomboidal 

 space, bounded on each side by the crura of the cerebellum. Its 

 floor is formed by the posterior (dorsal) surface of the medulla. Its 

 roof is formed by the cerebellum and by the very delicate layer of 

 nervous matter placed between the processus a cerebello ad 

 testes, and already spoken of as the valve of Vieussens. It is also 

 bounded by a still more delicate film of nervous substance which 

 extends backwards from the cerebellum between its posterior (or 

 inferior) crura, the restiform tracts. 



This ventricle is prolonged onwards by a narrow passage into a 

 larger cavity, the third ventricle, from the anterior wall of which a 

 small aperture leads right and left into two lateral ventricles (one on 

 each hemisphere,) each of which is still further continued on into 

 the olfactory lobe in front of it. The further relations of the 

 various parts will be best understood by studying a median, 

 vertical antero-posterior section of the brain. 



If the brain be thus BISECTED in the line of the longitudinal 

 fissure, we find as follows : 



The inner surface of the cerebral hemisphere in view is convo- 

 luted, and the cerebrum may be seen to extend forwards together 

 with and above the olfactory lobe in front, and beyond the anterior 

 end of the cerebellum behind. 



Beneath the middle of the cerebrum we come to the cut surface 

 of the corpus callosum, the front part of which bends rather sharply 

 backwards and downwards, forming what is called the knee (genii). 

 Beneath the bent-back extremity of the corpus callosum is the cut 

 edge of the lamina cinerea (or terminalis). At the upper part of this 

 lamina we find the cut surface of a transversely- extending white 

 cord, called the anterior commissure, and immediately behind the 

 lamina we find another cord, part of what is called thefomiv. This 

 latter structure extends, not transversely, but at first upwards and 

 forwards ; afterwards curving backwards it passes to the hinder part 

 of the corpus callosum. The fornix is the median part of ^ what is 

 really and morphologically the back of the cerebral hemispheres, 

 each half of the fornix belonging to one of the hemispheres. The 

 layer joining the two diverging and posterior portions of the 

 fornix is called the lyra, and together these parts form part of the 



