THE NERVOUS SYSTEM 345 



into its hemisphere, between the nucleus caudatus and optic 

 thalamus (the latter belongs to the " between-brain ") on the 

 inner side, and the nucleus lenticularis on the outer. In this 

 passage the compact crus, which is somewhat flattened, is 

 termed the " internal capsule." Immediately above the 

 three grey masses the internal capsule disperses as a fountain of 

 fibres which go to all parts of the cortex. Mingled with these 

 radiating fibres are vast numbers of others, proper to the 

 hemispheres, which run tangentially. Some, crossing the 

 median plane, as the corpus callosum, bind the two hemi- 

 spheres together. Others form tracts which can be followed 

 from one end or pole of the hemisphere to the other. Groups 

 of fibres, dipping but little below the cortex, unite nearly 

 adjacent spots or neighbouring convolutions. 



The folding of the cortex beneath fissures is due to the 

 necessity of disposing of a certain bulk of grey matter without 

 increasing its thickness beyond the proper limit. Since the 

 superficial area of a sphere varies as the square of its radius, 

 whereas its capacity varies as the cube, it is possible for a fixed 

 relation to be maintained between the amount of cortex and 

 the amount of white matter in the brain, only by the folds 

 increasing in depth as the size of the brain increases. Fissuring 

 is a response to a mechanical need. This does not imply, how- 

 ever, that the lines along which it takes place are devoid of 

 morphological meaning. The similarity in pattern of the con- 

 volutions and fissures in various animals, and the regular 

 progress of their development in each individual, prove the 

 contrary. If they are not absolutely trustworthy as boun- 

 daries of areas of separate function and further evidence will 

 be needed before a decision can be pronounced upon this dis- 

 puted question they are in the main satisfactory as landmarks. 



As the nervous system grows, the axons of its neurones 

 acquire their fatty (myelin) sheaths in the order in which they 

 come into functional activity. The passage through them of 

 impulses is the stimulus which leads to the deposition of fat. 

 The study of the progress of myelination enabled the anatomist 

 Flechsig to ascertain the situation within the brain of the 

 tracts of fibres related to the several senses, and hence the 

 traffic of the areas of the cortex to which they go. Glistening 

 white streaks appear successively in the pulpy yellowish-pink 



