154 MORPHOLOGICAL TYPES 



say, that in place of the two optic lobes of lower vertebrates, 

 there are now four corpora quadrigemina. 



The sides of the between-brain are thickened to form the 

 optic thalami, so much so indeed that the two sides touch one 

 another across the constricted 3rd ventricle, forming the 

 " soft commissure." The roof of the between-brain bears the 

 pineal stalk, the floor is depressed to form the infundibulum 

 to which the pituitary body is attached. Posterior to the 

 pituitary, the corpora mammillaria form two prominences 

 depending from the floor. The main bundles of fibres which 

 pass up and down from the brain and spinal cord run in the 

 ventral portion of the hindbrain, dorsal to the pons Varolii, 

 and diverge right and left in the region of the infundibulum 

 forming the crura cerebri. 



The cerebral hemispheres, or roofs of the lateral ventricles 

 forming the end-brain, are enormous and extend backwards 

 covering over the between-brain and midbrain. The super- 

 ficial layer of nerve-cells or grey matter forming the cerebral 

 cortex, which was slightly developed in reptiles, is in the 

 mammals thick and well formed. The surface is thrown into 

 a few folds, forming sulci and gyri ; but these are not so 

 numerous in the brain of the rabbit as in higher mammals. 

 The body of the hemispheres is marked out into a number of 

 lobes by fissures (frontal, parietal, occipital, and temporal 

 lobes). The two hemispheres are separated by a deep cleft 

 or median fissure, but the cortex of each side is connected with 

 that of the opposite side by a broad band of transverse fibres 

 forming the corpus callosum, likewise peculiar to mammals. 

 The cavities of the hemispheres are the lateral ventricles, which 

 communicate with the 3rd ventricle by the foramina of Monro. 



Beneath the temporal lobes are the pyriform lobes which 

 correspond to part of the roof of the end-brain of reptiles, and 

 which communicate with the olfactory lobes in front. The 

 floor of the end-brain is marked by the optic chiasma and the 

 corpus striatum. 



The various regions and centres of the brain in mammals are 

 extensively connected with one another by tracts of fibres. 

 Most of these connexions can only be made out by detailed 



