3 o2 SCIENCE PROGRESS 



eighteen. After sixteen the increase in brain weight is very 

 slight. In old age the brain tends to lose weight. 



Myelination and Preparedness for Function. — Now let me call 

 your attention to these diagrams after Flechsig (Plate IV, fig. 5) ; 

 see, the dots on these two diagrams are situated around the primary 

 fissures which physiological experiments and observations on 

 the brains of human beings suffering from disease show to be 

 the arrival and departure platforms of the sensory and motor 

 impulses. The portion of the brain where voluntary motor 

 impulses are generated for the control of movements of the 

 opposite side of the body lies in front of the central fissure ; 

 behind the central fissure is the central station for the reception 

 of impulses from the skin, muscles, joints, and tendons and 

 the general organic sensibility of the body. The half-vision 

 centre occupies the posterior part of the brain ; only a small 

 portion of this cortex is here seen because the greater portion 

 is deeply situated in the floor and walls of the calcarine fissure 

 on the mesial surface. The centre of hearing sounds received, 

 especially in the opposite ear, is also in great part hidden from 

 view, occupying the posterior part of the floor of the Sylvian 

 fissure ; likewise the cortex having for its function the sense of 

 smell is almost completely hidden; the sense is shown as 

 occupying a region at the tip of the temporal lobe. 



The Association Centres. —The portions of the cortex indi- 

 cated by dots situated around the primary fissures are, according 

 to Flechsig, the arrival and departure stations for afferent and 

 efferent stimuli. He terms them Projection Centres. But it 

 will be observed that the greater part of the surface grey matter 

 of the brain in Plate IV, fig. 5 shows no dots indicative of 

 projection systems ; these areas Flechsig terms the association 

 centres ; and although in man the afferent sensory and motor 

 efferent projection centres occupy a larger surface area than 

 in the highest anthropoid apes, it is especially the great 

 development of the association centres which accounts for the 

 fact that the cerebral cortex of a savage, even, is three times 

 as extensive as that of the gorilla. Now how do we know by 

 a study of the brain of the new-born child compared with 

 the brain at later periods of growth that the projection systems 

 are localised in the regions indicated ? I have already told you 

 that by appropriate staining the myelin sheaths of nerve fibres 

 can be detected in microscopic sections of the brain. I have 



