340 AN INTRODUCTION TO ZOOLOGY 



The meninges are readily studied in the sheep's brain, and with 

 a little care they may be removed from the cranium intact with the 

 "brain. The falx cerebri is easily recognisable, although it only 

 penetrates the sagittal fissure a very short distance save at its 

 posterior end, where it joins the tentorium, which is well developed. 

 In the thickness of these two folds lie venous sinuses which convey 

 the blood away from the brain. They are termed the sinus sagittalis 

 and the sinus transversus respectively. Turning now to the brain 

 itself we find it to be a large ovoid structure of which the anterior 

 three-quarters of the dorsal portion is constituted by the cerebrum. 



Fore-Brain. 



The large hemispheres are highly specialised outgrowths 

 from the dorsal region of the telencephalon, and they extend so far 

 back that they completely hide the thalamencephalon and mesence- 

 phalon. They are separated in the middle line by a well-marked 

 sagittal fissure, and their well-rounded surfaces are covered with 

 very distinct grooves or sulci dividing them off into a series of well- 

 marked broad ridges, the convolutions or gyri. Certain of these 

 sulci are of importance, since they serve to mark off the surface of 

 the brain into more or less well-defined areas. The first of these 

 is the fissura cruciata, which starts as a deep groove in the mid- 

 dorsal line and passes transversely, following a slightly curved course 

 and separates an anterior frontal lobe from a larger posterior 

 parietal lobe. The second is the fissura suprasylvia, which arises in a 

 Y-shaped union of two sulci just behind the fissura cruciata about 

 half-way out, and runs at first backwards and then outwaids and 

 downwards over the side of the hemisphere ; in its posterior two- 

 thirds it marks the boundary between the parietal and the descend- 

 ing temporal lobe. The third of them is the fissura rhinalis, which, 

 although visible from the side, is best seen from the ventral aspect, 

 as it starts just lateral to the olfactory lobe and passes backwards 

 with a slightly bowed course to the hinder end of the hemisphere. 

 The area on the inside of this is sometimes termed the pyriform lobe, 

 and its hinder end distinguished as the hippocampal lobule. All the 

 various grooves and ridges are formed by the folding of the wall of 

 the cerebellum, and they have the result of giving a greatly increased 

 surface. As has been noted before, the grey matter constituting 

 the cortex is the seat of the actual nerve cells, and so by this means 

 a much larger area is provided and so many more cells can be 

 accommodated. 



If the sagittal fissure be opened on the dorsal side, or if the median 

 edges of the hemispheres be sliced off, the corpus callosum will be 

 revealed. It is a transverse band of fibres occupying about two- 



