MAMMALIA. 301 



The brain is elongated, but not particularly well developed. 

 It presents the usual division into fore-, mid-, and hind-brain. 



Fore-Brain. The thalamencephalon possesses two parts not 

 present in preceding types. One is the middle commissure, a 

 broad band of grey matter connecting the side- walls of the third 

 ventricle between the anterior and posterior commissures. The 

 other is a rounded eminence, the corpus mammillare, seen on the 

 under surface immediately behind the pituitary body, which is 

 attached as previously to the infundibulum. With the hinder 

 part of the thin roof of the third ventricle the pineal gland is 

 connected by a hollow bifurcated stalk. 



The cerebral hemispheres are broad behind and taper in front 

 to blunt points. Externally they are almost smooth, but each is 

 divided by grooves into three lobes, frontal, parietal, and temporal, 

 placed respectively in front, postero-dorsally and postero-ventrally. 

 As in the frog and pigeon the hemispheres are closely applied in 

 the middle line, but here, in addition, they are firmly united by 

 an elongated band of transverse fibres, the corpus callosum. Below 

 this and continuous with it behind is another fibrous band, the 

 body of the fornix, which divides in front into four smaller bands, 

 the pillars of the fornix. 



Two of these, the anterior pillars, pass downwards in the lamina 

 terminalis and substance of the optic thalami to reach the corpus 

 mammillare, while the other two, posterior pillars, curve outwards into 

 the hemispheres. Each lateral ventricle is divisible into three parts a 

 narrow anterior cornu in front, and towards the inner side a posterior 

 cornu, similarly placed behind, and a descending cornu, which passes 

 down into the temporal lobe. A prominent oval mass, the hippocampus 

 major, projects into the last-named cornu, and along the anterior edge 

 of this the posterior pillar of the fornix runs. In front of this pillar 

 there is a vascular fold, the choroid plexus, which extends into the ven- 

 tricle from the pia mater. The corpus striatum forms the outer side and 

 floor of the anterior cornu. The cerebral hemispheres fuse where they 

 meet in front of the lamina terminalis, and form a party-wall, the septum, 

 lucidum, to the hinder parts of the anterior cornua, immediately in front 

 of the foramen of Monro on each side. This partition is not solid, but 

 contains a small slit-like fifth ventricle. 



This is not, therefore, like the other ventricles, a part of the cavity of 

 the original neural tube of the embryo. These were originally named 1st 

 and 2nd (i.e., the two lateral), 3rd and 4th ventricles, and when another 

 brain-cavity was discovered it naturally received the name 5th ventricle. 



The olfactory lobes are elongated and dilated at their ends. 

 They are attached to the under sides of the frontal lobes, and 

 run forwards in front of them to rest on the cribriform plate. 



