412 PHYSIOLOGY 



brain stem to be distributed throughout the whole of the cerebral hemi- 

 spheres. This forms a sort of capsule to the optic thalamus, lying between 

 this body and the corpus striatum behind, but in front piercing the corpus 

 striatum between its two nuclei. It is called the internal capsule. 



The development of the different parts of the brain stem from the 

 three cerebral vesicles and their gradual subordination and over- 

 shadowing in the course of development by the cerebral hemispheres 

 is well shown if we compare the brain of a fish with that of a reptile 

 and again with that of a mammal (Fig. 181). Man's position in the 

 scale of animal life is determined, not by increasing complexity of the 

 structures forming his brain stem, but by the gradual subordination 



17 



\ '\-;&*l ^BSOTKS^A. / i ISS&ftffO /'*. 



-X. ~ IT/ ^^^^^ / 

 C.a. 



FIG. 182. Section through the lower border of the medulla oblongata, at 



the pyramidal decussation. (BECHTEREW.) * 



fla, anterior fissure ; d, decussation of the pyramids ; V, anterior columns ; 

 Co, anterior cornu ; cc, central canal ; S, lateral columns ; fr, f ormatio reti- 

 cularis ; ce, neck, and g, head of the posterior cornu ; rpCl, posterior root 

 of first cervical nerve ; nc, beginning of nucleus cuneatus ; tig, nucleus gracilis ; 

 H 1 , f uniculus gracilis ; H 2 , funiculus cuneatus ; sip, posterior fissure. 



of these to the latest formed cerebral hemispheres, and the enormous 

 growth of his capacity to adapt himself to a varying environment 

 consequent on the increase in size of his cerebral hemispheres. 



THE HIND-BRAIN 



It will be convenient to trace first the modifications undergone 

 by the axial part of the nervous system in the brain, and then to 

 deal with the new masses of grey matter which have no homologies in 

 the spinal cord, as well as the long tracts of white matter serving to 

 connect different levels or different sides of the brain. 



In examining successive sections from the spinal cord up through 

 the medulla, the first change which makes its appearance is due to the 

 decussation of the pyramids (Fig. 182). 'Throughout the spinal cord, 



