NERVOUS SYSTEM 461 



These now occupy only small areas — the pyriform lobe on the 

 ventro-lateral aspect, and the hippocampus, which has been 

 tucked in on the median side and is now mainly internal. With 

 the development of the neopallium is connected the increased 

 power of co-ordination possessed by the highest vertebrates, 

 and the great extent of this region in mammals is accompanied 

 by the ability to meet new situations by new behaviour. 



The remaining part of the fore-brain, the thalamencephalon, 

 is overhung and hidden by the cerebral hemispheres. Its thick 

 sides form two large thalami, and from the hinder part of its 



a.cji 

 3 ^;^- .'p.c.q 



..\ /--r-j !ph 



cer.hr-^ 



c.c 



\ 



ol.b. 



II 



o/c. '^/ 



I 



pit 



, c.al\ 



'cer.h. \ \rri.oh. 

 pXi 



Fig. 362. — A semi-diagrammatic, median, longitudinal section of the brain of a 



rabbit. 



aq., Aquaeductus Sylvii ; ex., corpus callosum ; inf., infundibulum ; m.c, middle commissure, which 

 connects the two optic thalami across the third ventricle ; o.c, optic chiasma ; p.ch.p., posterior 

 choroid plexus ; 3, 4, ventricles. Other lettering as in Figs. 360, 361. 



thin roof the pineal stalk passes backwards to end in the pineal 

 body between the hinder ends of the hemispheres. The infundi- 

 bulum is a funnel-like depression of the floor of the thalamen- 

 cephalon, which enters the pituitary body. The latter, with the 

 bottom of the infundibulum, is usually torn off in removing the 

 brain from the skull, leaving a longitudinal slit which leads into 

 the third ventricle or cavity of the thalamencephalon. A small, 

 rounded, median swelling immediately behind the infundibulum 

 is known as the corpus mammillare or corpus albicans. The mid- 

 brain is almost covered by the cerebral hemispheres. Behind each 

 of its optic lobes there is one of auditory function, so that four 

 corpora quadrigemina or colliculi are seen. The floor and walls 

 of the mid-brain are prominent crura cerebri. The anterior part of 

 the hind-brain, the cerebellum, is very large and much folded 

 and consists of a median lobe or vermis and two lateral lobes, 



