530 THE RABBIT CHAP. 



its hinder part arises a stalk bearing at the end a small, 

 rounded pineal body (pri) . The floor of the diencephalon 

 is produced downwards to form the infundibulum (inf), to 

 which the pituitary body (pty) is attached. In front of 

 the infundibulum is the optic chiasma (o. ch), and behind 

 it a small, rounded lobe (c. ma}. 



Each optic lobe is divided into two by a transverse 

 furrow, so that there are four rounded elevations in this 

 region an anterior, larger pair (o. I 1 ), and a posterior, 

 smaller pair (o. I 2 ). Below the optic lobes are the 

 crura cerebri (c. c) two strong, diverging bands passing 

 forwards and outwards from the bulb to the 

 hemispheres. 



The bulb or medulla oblongata (m. o) is slightly flattened 

 dorso-ventrally, and passes behind into the spinal cord, 

 the dorsal and ventral fissures of which are continued 

 into it : the fourth ventricle (t> 4 ) which it contains is roofed 

 over by the thin pia mater only (p. 155). Ventrally its 

 anterior border is marked by a stout band of nerve-fibres 

 running transversely, and known as the pons Varolii (p. 

 va). The large cerebellum is connected with the dorsal 

 surface of the brain by three pairs of peduncles (Fig. 141, 

 a. pn, p. va, p. pn), and consists of a median central lobe 

 (cjb 1 ) and of two lateral lobes (cb 2 ), on the outer side of 

 each of which is a smaller floccular lobe (fl) . The grey 

 matter is superficial, and the surface is marked by 

 numerous folds which in section present a tree-like 

 pattern (arbor vita], brought about by the arrangement 

 of the grey and white matter (Fig. 142). 



The fourth ventricle is not prolonged into the cere- 

 bellum to any extent : it is continued forwards as the 

 iter, from which no optic ventricles are given off (com- 

 pare pp. 157 and 459) and which passes into the narrow 

 but deep third ventricle in front (Fig. 142) : this is 

 bounded anteriorly by a thin wall, the lamina terminalis 



