34 Anatomy op the; Rabbit. 



the sole of the foot rests on the ground. Thus the muscles described as 

 flexors in the foot pass over the heel. They are functionally flexors of 

 the toes, but extensors of the foot as a whole. Those described as 

 extensors, lie for the most part on the anterior surface of the tibia, and 

 are turned from their original course to the dorsal surface of the foot. 

 Thev are functionally extensors of the toes, but flexors of the foot as a 

 whole. 



THE NERVOUS SYSTEM. 



In the central nervous system, the more general features relate to the 

 form of the whole structure as a neural tube, containmg a central cavity, 

 the neurocoele. This is differentiated into two portions, namely, a less 

 modified portion, the spinal cord, or spinal medulla, containing as its 

 cavity the central canal, and a greatly expanded portion, the brain, or 

 encephalon, which is divided into a series of paired and unpaired seg- 

 ments, and contains in its interior a corresponding series of ^divisions of 

 the original cavity, known as its ventricles. 



Although the internal structure of the brain is a matter of the dis- 

 position of its nerve-centres and fibre-tracts, it is customary to estimate 

 its primary divisions by reference to a general plan, the latter being in 

 most respects one that is common to vertebrates generally and thus 

 appears in a less elaborated condition in the embryo, or in the adults of 

 lower forms. The main features of this plan are indicated in the ac- 

 companying diagram (Fig. 23). 



The brain as first formed in the embryo appears as an anterior ex- 

 panded portion of the neural tube, or rather three expansions 

 arranged in a linear series. They are described as the primary cerebral 

 vesicles; or, as primary divisions of the future brain, they are designated 

 in anteroposterior order as the prosencephalon, mesencephalon, and 

 rhombencephalon. 



The first of the primary divisions, the prosencephalon, or primary 

 fore-brain, becomes divided during development into two portions, 

 namelv, an anterior portion, the end-brain or telencephalon, which is 

 largely a paired structure, and a second portion, unpaired, the dience- 

 phalon, or inter-brain. The larger paired portion of the telencephalon is 

 the basis of the cerebral hemispheres. It contains, as divisions of the 

 primary cavity, a pair of cavities, the lateral ventricles. The anterior 

 portion of the telencephalon, moreover, becomes differentiated, so that a 

 small terminal olfactory segment, the rhinencephalon, is more or less 

 perfectlv marked off from the rest. In the mammalian brain this part 

 is chiefly identifiable as the paired olfactory bulb, the latter being the 

 anterior portion of the olfactory lobe or olfactory brain, and containing 

 in its interior an extension of the lateral ventricle. 



The unpaired portion of the prosencephalon is considered as belonging 

 in part to the telencephalon and in part to the diencephalon. Its 

 cavity, the third ventricle, is connected with the lateral ventricles through 

 the interventricular foramen. Its anterior wall is formed by a transverse 

 connection of the cerebral hemispheres, the lamina terminalis. In all 



