364 



PHYSIOLOGY 



thickened and consist of white matter internally and grey matter externally. The part 

 of the hemisphere which lies over the first cerebral vesicle is undifferentiated and remains 

 as a simple epithelial layer. This becomes closely applied to the similar layer forming 

 the roof of the third ventricle, from which it is separated only by a process of the pia 

 mater carrying numerous blood-vessels (the velum interpositum). In the adult brain the 

 cavities of the cerebral hemispheres are known as the lateral ventricles, the remains 

 of the first cerebral vesicle receiving the name of the third ventricle. The lower and outer 

 part of the hemispheres, i.e. the part which is first formed, becomes much thickened and 

 forms the corpus striatum, which is closely applied to the front and outer part of the 

 optic thalamus. In the corpus striatum two masses of grey matter are developed, 

 namely, the nucleus caudatus and the nucleus lenticularis. A layer of nerve fibres ascends 

 from the brain stem to be distributed throughout the whole of the cerebral hemispheres. 

 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 overshadowing 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. 180). 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 of these to the latest formed cerebral 

 hemispheres, and by 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 



cc 



IM. Section through the lower border of the medulla oblongata, at the 



pyramidal decussation. (BECHTEREW.) 



//". anterior lisaure ; rf, decnssation of the pyramids ; I', anterior columns ; 

 mi : -, . , -run-ill canal : N, lateral columns : //. fonnatio reti- 

 ularis ; 06, neck. ;m'<l ,,. |,, .,-,,! of the posterior rorini : //><'/, posterior root of 

 first cervical nerve ; ,-. he^innini: of nucleus cuneatus : //,/, nucleus gracilis ; 

 //'. fiiniciilus graoflis; //-. funiculu^ .unriitiis ; ,s//,. posterior tissure. 



as the long tracts of white matter Serving to connect different levels 

 or different sides of the brain. 



