70 ANATOMY OF THE RABBIT. 
tion. The anterior tips of the cerebral hemispheres in the rabbit 
are slightly 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 
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, not 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, the 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- 
THE BRAIN. 
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 
