372 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



The form of the cerebral cortex, which resembles a cap 

 covering the outer surface of the hemispheres, appears specially 

 fitted for this comprehension of the conductive tracts (figs. 

 230, 231, and 232, F, 0, Tp, R, H}. This form results from the 

 grouping of the innumerable sensory elements occupying the 

 cortex, namely, the nerve cells. The sensory nerves constitute 

 their feelers, the motor their arms. Since this convoluted mass 

 of fibres must in great part pass through the occipital foramen 

 in order to reach the several organs, there is a convergence of 

 them from all sides, both in the peduncles of the brain and in 

 the spinal cord, towards the grey matter of the central cavities. 

 After however they have traversed this central grey substance, 

 they diverge, as the peripheral nervous system, to all parts of 

 the body. Since now this organization effects the contact of 

 the sensory shell of the cortex of the cerebrum with the various 

 forms of sensory impressions derived from the external world, 

 the image of which is coincidently projected upon the cortex, 

 the name of projection system is very appropriate to this great 

 segment of the nervous system, and in this comparison the cor- 

 tex of the cerebrum is to be regarded as the surface on which 

 the projection is received, while the external world stands for 

 the projected object (P 1 P 2 P 3 ). 



As the movements of the body constitute the source of cer- 

 tain kinds of sensation (muscular sensations), so do they also 

 form a part of the external world projected upon the brain. The 

 muscular system, however, is still in another sense of the word 

 a projected object ; namely, through the centric and peripheric 

 tracts of the motor nerves, along which the cerebral cortex 

 reacts again upon the outer world in response to the condition 

 of excitation induced in it by its sensory nerves. 



In fig. 230, P! P 2 P 3 represent the successive links of the 

 projection system, which last is interrupted several times by 

 masses of grey matter. The first and uppermost link (P 1 P/ 

 and Br) is a medullary system springing for the most part in a 

 radial manner from the cortex, and extending peripherically 

 into the second principal mass of grey substance, the ganglia 

 at the base of the hemisphere (corpus striatum and optic 

 thalamus) (figs. 230, 231, 232, Cs, Th, Qu). From the inter- 

 rupting network of the ganglionic masses springs the second 





