DISSECTION OF THE BRAIN, OR ENCEPHALON. 249 



great sheet of grey raatter on the surface of the hemisphere invests 

 the white core in each conA^olution, and also extends across the bottom 

 of each fissure. 



The Corpus Callosum (Plate 35 and Fig. 31) is a great commissure of 

 nerve fibres connecting the right and left hemispheres. It termin- 

 ates behind in a thickened max-gin — the sjylenium; and in front it is 

 abruptly bent downwards and backwards, the bend being named the 

 genu, and the reflected portion the rostrum. The rostrum becomes 

 narrower as it descends, and is connected to the optic commissure by the 

 lamina cinerea. Along the middle line of its lower face the corpus 

 callosum is connected posteriorly with the fornix, and anteriorly with 

 the septum lucidum ; and on each side it forms the roof of a cavity in 

 the hemisphere — the lateral ventricle. Nearly all the fibres of the 

 corpus callosum have a transverse direction ; but on each side of the 

 longitudinal middle line of its vipper face there are a few longitudinal 

 fibres termed the striae longitudinales, or nerves of Lancisi. 



Directions. — If the corpus callosum be now cut through in the longi- 

 tudinal direction, a little to one side of the middle line, and dissected 

 outwards, the lateral ventricle will be exposed. The corpus callosum, 

 it will now be seen, is thickest at its posterior extremity, and thinnest at 

 its middle. 



The Lateral Ventricles (Plate 35) are two in number, one in each 

 hemisphere. They are separated from one another along the middle 

 line by the fornix and septum lucidum, but beneath the former body they 

 communicate through the foramen of Monro. The central portion of 

 each cavity is termed the body, and its prolongations before and behind 

 are termed respectively the anterior and the descending cornu. 



On the floor of the body of the cavity the following objects will 

 be noticed : — In front, a large pear-shaped grey eminence — the corpus 

 striatum ; behind, another body of about the same size but white on its 

 surface — the hippocampus; between the corpus striatum and the 

 hippocampus, a groove, in which there lies a red granular cord — the 

 choroid plexus. Where the hippocampus bounds this groove, it is 

 margined by a white band — the tamia hippocampi ; and if the choroid 

 plexus be pulled gently backwards, another white band will be seen to 

 margin the corpus striatum where it bounds the groove— this is the 

 tcenia semicircularis. * 



The anterior cornu is occupied by the base of the corpus striatum. 

 It curves downwards and forwards into the anterior part of the 

 hemisphere, where it communicates with the cavity of the olfactory bulb. 



The descending cornu contains the prolongations of the hippocampus 

 and its taenia. It passes at first backwards and outwards, and then 



* The optic thalamus and tsenia semicircularis are generally enumerated among the objects 

 visible in the body of the lateral ventricle. In the brain of the horse, however, the choroid plexus 

 completely conceals from view the optic thalamus, and in most cases also the taenia semicircularis. 



