80 



ANATOMY OF THE RABBIT 



of its outstanding features, and below by the somewhat tapering 

 medulla oblongata, which is continuous caudally with the spinal 

 cord. On the ventral side, the medulla oblongata is crossed in front 

 by a bridge of fibres, not so conspicuous in the rabbit as in many 

 mammals and in man, which is known as the pons and which 

 appears to connect the two sides of the cerebellum. (Actually it 

 is part of the path to the latter from the cerebral hemisphere.) 

 These, the outstanding surface features of the brain, afford but a 

 moderate conception of its details, the nature of which can be made 

 out only 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 en- 

 larged and elaborated to include both the highest controlling 

 centres of the whole nervous mechanism and the special centres of 

 the nervous mechanism for a variety of functions performed by 

 organs in the head. It accordingly not only forms a more or less 

 distinct division, known as the brain or encephalon, as opposed to 

 the less elaborated spinal cord or spinal medulla, but also develops 

 a series of paired and unpaired subdivisions containing portions 

 of the original cavity distended to form ventricles. 

 The primary divisions of the brain are more or 

 less similar and homologous in all vertebrates. The 

 more elaborate condition of the organ in a mammal 

 may be explained by reference to the general plan 

 as indicated in Fig. 44, which is based upon general 

 features of form in vertebrates and upon embryonic 

 development. For comparison in the gross, the 

 brain of the frog (Fig. 43) offers one of the best 

 examples. 



The brain as first formed in the embryo appears 

 as three anterior expansions of the neural tube 



Fig. 43. The , . ,. ^ . _, , ., , 



brain of the frog arranged m a Imear series. Iney are aescribea as 



from the dorsal , . i i • < • i- 



surface: c, cerebei- the primary Cerebral vesicles; or, as pnmary di- 



phTi'on; fv, fourth visious of the futurc brain, they are designated in 



brai^'^ hemisphere; anteroposterior order as the prosencephalon, mes- 



oi. opt?? lobe. ^"'^' encephalon, and rhombencephalon. 



