346 THE BODY AT WORK 



substance of the interior of the brain. At the time of birth 

 all the fibres which enter or leave the cerebral hemispheres have 

 acquired their myelin sheaths. In the baby's brain the sense- 

 organs have established all their connections with the cortex. 

 No new fibres will appear in the nerves of the eye, the ear, or 

 the other sense-organs, nor will their end-stations in the cortex 

 be further multiplied. (The use of the expression " end- 

 stations " is legitimate so far as sensations are concerned ; 

 notwithstanding that all sensory impulses are retransmitted 

 by neurones in the cerebro-spinal axis.) But the cortex is very 

 far from having finished its growth. It contains a large amount 

 of embryonic tissue, which gradually spreads outwards from 

 the developed areas into the surrounding unoccupied zones. 

 The taking up of new territory, and the consequent increase 

 in the size of the brain, is continued into adult life. The study 

 of progressive myelination enabled Flechsig to divide the cortex 

 into "sensory centres," and intervening "association zones"; 

 although, doubtless, the difference in function between the 

 portions which receive sensations direct and the portions in 

 which the products of sensation are worked up is one of degree, 

 and not of kind. 



The structure of the cortex is not quite the same in sensory 

 and association areas ; but it is everywhere so far from showing 

 the diagrammatic simplicity which characterizes the cortex of 

 the cerebellum as to make it difficult to summarize the modifi- 

 cations which distinguish its various regions. To a considerable 

 extent its elements shade one into the other, differing in size 

 and in orientation rather than in form. Commonly it is de- 

 scribed as divisible into five layers : (1) A thin superficial layer, 

 containing cells of various forms and fibres derived from the 

 cells of the deeper strata. Some of the cells are pluripolar, 

 possessing several axons which run parallel with the surface. 

 Their destination is unknown. They do not appear to form 

 baskets like the cells of the molecular layer of the cerebellum. 

 The dendrites of pyramidal cells extend into this layer. (2) The 

 layer of small pyramids ; cells with a branching apical process, 

 root-like dendrites from the basal angles of the pyramid, and 

 an axon which sinks into the white matter. (3) Granules. 

 Carmine or other nuclear stains show that small cells are 

 present in very large numbers, especially in the sensory areas ; 



