V 



Nervous System General, 123 



mant of the primitive cavity of the sac : but, in Mammals, 

 the wall of the vesicle is augmented by folds, of which the 

 first and most constant is pushed from the mesial or inner 

 side of the ventricle into its cavity, giving rise to the con- 

 vexity, representing the part called ' hippocampus* in Anthro- 

 potomy. The anteriorly continued and transversely connected 

 longitudinal fibres of the Hippocamp constituting the Fornix, 

 furnish a further distinctive characteristic of the Mammalian 

 as compared with the Avian brain. An intercommunication 

 between the two Prosencephalic cavities (lateral ventricles) 

 defined in Anthropotomy as the foramen of Monro, exists in 

 all Mammals. The extent to which the Hemispheres are 

 developed posteriorly is very variable, being in some genera 

 so slight as to leave the Corpora Quadrigemina exposed, and 

 being seldom sufiicient to entirely cover the Cerebellum. 

 In proportion to the increase of the Cerebral Hemispheres 

 there is a diminution in the size of the ganglia immediately 

 connected with the organs of sense. The external surface of 

 the Hemispheres is generally convoluted, but there are several 

 Orders and Genera in which the surface is smooth, as it is in 

 the brain of the Oviparous Yertebrata. 



4. The foremost primary division of the Brain, termed the 

 Rhinencephalon, includes the anterior termination of the co- 

 lumnar tracts (the * roots of the olfactory nerves' in Anthro.), 

 and the appended vesicular mass called 'olfactory lobe' (the 

 *bulb of the olfactory nerve' in Man). In most Mammals 

 the * lateral ventricles' of the Prosencephalon are continued 

 into the Rhinencephalon. As a very rare -exception the 

 * olfactory bulbs' may be absent*. 



Membranes of the Brain. A fold of dura mater is commonly 

 found between the Cerebral Hemispheres termed the ' Falx '; 

 a second duplicature is found between the Cerebrum a^^'^ Cere- 



* In a few Cete, e,g. Dolphin. 



