Chap. XI.] THE BRAIN AND NERVES. 219 



are large, and united (1) by a posterior commissure, on either 

 side of which are the peduncles of the pineal gland ; and (2), 

 by a middle or soft commissure of grey matter (m. c.). The roof 

 forms a thin velum interpositum. 



In this region the central canal expands to form the third 

 ventricle of the brain (iii.). 



5. Prosencephalon, or secondary fore-brain. In the frog the 

 cerebral hemispheres (c. h.), composed externally of grey matter 

 and internally of white, are formed as outgrowing buds from the 

 anterior end of the primitive fore-brain. In their adult form 

 they are elongated anteriorly, but do not extend backwards over 

 the succeeding parts of the brain. They contain lateral ven- 

 tricles which communicate with the third ventricle by the foramen 

 of Monro "(/ m.). The outer wall of each lateral ventricle is 

 somewhat thickened. The inner faces of the two hemispheres 

 are united anteriorly in the region of the so-called olfactory 

 lobes. The anterior wall of the brain in the. mid-line is the 

 lamina terminalis (I. ), which contains a band of transverse 

 fibres, the anterior commissure (a. c.). In the pigeon the cerebral 

 hemispheres (c. h.) are rounded and smooth, and extend back- 

 wards, so as to separate the optic lobes and cover much of the 

 thalamencephalon. The cavity of the lateral ventricle is much 

 diminished, the floor and lower part of its outer wall being 

 thickened by the development of an eminence of grey matter 

 forming the corpus striatum. Above the lamina terminalis (/. t.) 

 is a band of transverse fibres connecting the corpora striata, and 

 forming the anterior commissure (a. c.). 



In the rabbit the early development of the cerebral hemi- 

 spheres (c. h.) resembles that in the frog. In their later develop- 

 ment matters become much more complicated. Externally they 

 are seen to be divided by a ventral and longitudinal groove into 

 a larger superior and a smaller inierior> lobe. A slight angula- 

 tion on the ventral side of the superior lobe marks the position 

 of that which becomes in higher types the fissure of Sylvius. 

 The hemispheres grow rapidly, and extend backwards so as to 

 cover the thalamencephalon, and almost hide the optic lobes 

 (nates and testes). The floor becomes thickened, giving rise to 



