70 Anatomy of the Rabbit. 



tion. The anterior tips of the cerebral hemispheres in the rabbit 

 are sHghtly expanded to form the olfactory bulbs, and the con- 

 nection of the latter with the posteroventral portion of the hemi- 

 sphere known as the piriform lobe is easily traced. This portion 

 of the brain is chiefly of interest in mammals because of its relation 

 to the olfactory sense organs and because of the reduced condition 

 of the organ in man. The development of the cerebral hemispheres 

 _ Dp ATM- even in the lower mammals is such that important 

 parts of the brain, notably the diencephalon and 

 mesencephalon, are dorsally and laterally concealed. On the 

 ventral side, however, the chiasma or crossing of the optic nerves is 

 evident, while more posteriorly the divergent cords of the cerebral 

 peduncles may be seen passing forwards in the direction of the 

 hemispheres. The posterior part o the brain is formed largely 

 by the cerebellum above, the corrugation of which is one of its 

 outstanding features, and by the somewhat tapering brain -stem 

 by which the brain is connected with the spinal cord. Its bulk is 

 formed by the medulla oblongata, which is crossed in front by a 

 bridge of fibres, riot so conspicuous in the rabbit as in many mam- 

 mals and in man, which is known as the pons and which connects 

 the two sides of the cerebellum. These, ths outstanding surface 

 features of the brain, afford but a moderate conception of its 

 details, the nature of which can only be made out by more thorough 

 examination and by reference to the plan of development of the 

 organ as a whole. 



Like the spinal cord, with which it is continuous, the brain forms 

 primarily a portion of the neural tube, containing a central cavity 

 or neurocoele, but unlike the spinal cord it is greatly enlarged and 

 elaborated both as the general centre of the whole nervous mechan- 

 ism and also the special centre of the nervous mechanism for a 

 variety of functions connected with the head. It accordingly forms 

 not only a more or less distinct division, known as the brain or 

 encephalon, as opposed to the more general division, the spinal 

 cord or spinal medulla, but is also divided into a series of paired 

 and unpaired segments, containing divisions of the original cavity 

 in the form of ventricles. 



The primary divisions of the brain are more or less similar and 

 homologous in all vertebrates. The more elaborate condition of the 



