X BRAIN 513 



hinder part of which arises a stalk bearing at its end a 

 small, rounded pineal body {pn). The floor of the 

 diencephalon is produced downwards to form the 

 infundibuluin {inf), to which the. pituitary body (pty) is at- 

 tached. In front of the infundibulum is the optic chiasma 

 (p. ck) 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 {p. /.^),and a posterior, smaller pair {0. l.^). 

 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, {in. 0) 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 (»*) 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 cerebellum is connected with the dorsal surface of 

 the brain by three pairs of peduncles (Fig. 131, a. pn, p. va, 

 p. pn), and consists of a median central lobe (cb.^) and of 

 two lateral lobes {cb.% on the outer side of each of which 

 is a smaller floccular lobe {ft). The grey matter is super- 

 ficial, and the surface is marked by numerous folds 

 which in section present a tree-like pattern {arbor vitce), 

 brought about by the arrangement of the grey and white 

 matter (Fig. 132). 



The fourth ventricle is not prolonged into the cerebellum 

 to any extent : it is continued forwards as the iter, from 

 which no optic ventricles are given off (compare pp. 151 and 

 443) and which passes into the narrow but deep third 

 ventricle in front (Fig. 132) : this is bounded anteriorly by 



Pract. ZOOL, J^ h 



