LEPUS CUNICULUS 329 



present the shape of rounded triangles with their blunt apices 

 directed forward. Internally they are hollow, containing the lateral 

 ventricles which communicate posteriorly through the foramina of 

 Munro with the third ventricle. Externally their convexly rounded 

 surfaces are 'markedly shallow grooves, the sulci, one of which, 

 lying at the side, is rather more marked than the remainder and 

 divides the hemisphere into an anterior frontal lobe and a posterior 

 parietal lobe, from which a ventral descending area, the temporal 

 lobe, is marked off by another distinct sulcus. The two hemi- 

 spheres are separated in the mid dorsal line by the deep sagittal 

 fissure, in which is the falx cerebri, and if they are pressed apart it 

 will be seen that a little way down their posterior ends are held 

 together by a broad transverse band of tissue, the corpus callosum, 

 which joins the nervous centres of the two sides of this part of the 

 brain. This band of fibres is peculiar to the higher mammals, not 

 being found outside that class, and it is even absent in the two 

 lowest orders of the Mammalia, viz. the Monoteremes and the 

 Marsupials. On the anterior ventral surface of the telencephalon 

 are two club-shaped bodies, the olfactory lobes, whose swollen 

 extremities project beyond the front end of the cerebral hemispheres. 

 The first pair of cranial nerves, the olfactory, leave their anterior 

 surfaces as a series of fibres. 



The next portion of the brain, the thalamencephalon, contains 

 the third ventricle, and is roofed by the anterior choroid plexus, 

 but it is not visible from the dorsal side, since it is entirely overlain 

 by the posterior ends of the hemispheres. The epipbysis cerebri or 

 pineal body coming from its postero-dorsal surface can sometimes 

 be made out between the divergent posterior lobes of the cerebrum. 

 On the ventral surface of this region of the brain we find the optic 

 chiasma, the infundibulum and the pituitary body or hypophysis 

 cerebri. In spite of its small size and inconspicuousness we shall 

 find when we come to consider the details of its structure and 

 relationship that it is a most important part of the brain. 



The mesencephalon or mid brain is also largely hidden by the 

 hemispheres, but can be seen easily if their hinder ends are pushed 

 apart. On the dorsal side are two pairs of rounded bodies, the 

 corpora guadrigemina or optic lobes, homologous with the corpora 

 bigemina of the dogfish or frog, but subdivided. The ventro- 

 lateral part of this region is constituted by the crura cerebri, two 

 smooth bands of fibres passing, backwards and the hollow within it 

 is the iter a tertio ad quart um ventriculum. 



The cerebellum is the well-developed dorsal region of the meten- 

 cephalon. It forms a transversely elongated mass consisting of a 

 median lobe, the vermis, and two lateral or floccular lobes, which 



