244 SPECIAL PHYSIOLOGY. 



In front of the pituitary body, the optic tracts of the two sides 

 coalesce, or decussate, to form the optic commissure, from which the 

 right and left optic nerves then proceed forwards to the eyeballs. It 

 remains to be added, that in the interior of the cerebral peduncles, 

 which are composed of white substance externally, there is also dif- 

 fused a large quantity of gray or ganglionic nerve substance. 



The ventricles of the cerebrum, mentioned so frequently above, are 

 five in number, and were, by the old anatomists, considered of special 

 importance, and to be the residence of what were then called the ani- 

 mal spirits; but they are really the remains of a simple cavity, formed 

 by the folding back of the hemispheres, in the progress of their devel- 

 opment, and gradually complicated in shape, owing to the projection 

 of the corpora striata and optic thalami into them, and to the exten- 

 sion of the white cerebral substance, in various directions, round about 

 them. As already stated, they are roofed in by the corpus callosum, 

 and by its lateral extensions into the hemispheres; whilst in front, 

 at the sides, and below, there are the corpora striata and optic thala- 

 mi, the cerebral peduncles, and certain layers of nervous substance 

 connecting those parts. The two largest chambers of these ventricu- 

 lar cavities, are the two lateral ventricles, right and left, one belonging 

 to each hemisphere. Each lateral ventricle presents a central part or 

 body of the cavity, and three prolongations named the cornua or horns, 

 viz., an anterior cornu or horn, which passes into the frontal lobe, a 

 middle or descending cornu, which curves backwards and outwards, 

 and then downwards, forwards, and inwards, into the temporal lobe, 

 and the posterior cornu, which passes backwards and outwards, and 

 then inwards, in the occipital lobe. The descending cornu contains, 

 besides the posterior ends of the fornix already mentioned, and other 

 parts, a projection or ridge in its floor, called the hippocampus major ; 

 and in the posterior cornu, is a similar smaller projection named hip- 

 pocampus minor ; between them, is the eminentia collateralis. Both 

 of these so-called hippocampi are merely portions of the hemisphere, 

 projecting into the ventricle, and corresponding with the bottom of 

 certain more or less well-marked fissures, or sulci, on the surface. 

 The third ventricle is situated in the middle line, near the base of the 

 brain, between the optic thalami ; it communicates with both the lat- 

 eral ventricles, and with the fourth ventricle to be presently noticed. 

 The fifth ventricle is a small independent cavity, situated in a septum 

 of nervous substance, found between the two lateral ventricles. 



The cerebellum. This part of the encephalon rests upon the occipi- 

 tal bone behind the foramen magnum, and is covered by the tentorium. 

 It consists, like the cerebrum, of two hemispheres, which, however, 

 are more extensively united than those of the cerebrum, and in a dif- 

 ferent manner, by a median portion, similarly constructed to the 

 hemispheres themselves, and forming, on the upper and under surface, 

 the so-called middle lobe, or superior and inferior vermiform, or worm- 

 like processes; a slight notch marks off the hemispheres in front and 

 behind. Each hemisphere is composed of smaller parts or lobes, sepa- 

 rated from one another by deep crescentic fissures. These lobes, as 

 well as the vermiform processes, are highly subdivided on their sides 



