834 THE CENTRAL NERVOUS SYSTEM 



this and the primitive grey stem are interposed (a) the sheath of 

 white fibres that clothes the latter, and connects its various parts, 

 and (b) a new development of white matter (corona radiata, cere- 

 bellar peduncles), which serves to bring the cortex into relation 

 with the primitive axis, and through it with the rest of the body. 



Although there are histological and developmental differences 

 between the cerebral and the cerebellar cortex, we may, for some 

 purposes, classify them together as cortical formations. And we 

 may also include under this head the corpora striata, which, although 

 for descriptive purposes generally grouped with the optic thalami 

 and the other clumps of grey matter at the base of the brain, as the 

 basal ganglia, are to be regarded as cortical in character. As we 

 mount in the vertebrate scale the cortex formation of the secondary 

 fore-brain and hind-brain acquires prominence. 



In other words, the grey matter developed in the roof of the cerebral 

 vesicles I. and III. (Fig. 317) (the grey matter of the cerebral and cere- 

 bellar cortex) comes to overshadow the superficial grey matter hitherto 

 present only in the roof of veskle II. (in the corpora bigemina). And 

 this cortex formation becomes larger in amount, and, in the case of 

 the cerebral grey matter, more richly convoluted, the higher we ascend, 

 until it reaches its culmination in man. As the anterior cerebral 

 vesicles develop, they spread continually backward, until at length the 

 cerebral hemispheres cover over, and almost completely surround, the 

 primary fore-brain and the mid- and hind-brains, so that the anterior 

 portion of the primitive stem comes, as it were, to be invaginated into 

 the second wider tube of cortical grey matter. This development of 

 the cortical grey substance is accompanied with a corresponding 

 development of nerve-fibres, for an isolated nerve-cell (apart, of course, 

 from possible embryonic rudiments which have not undergone com- 

 plete development) is no more conceivable than a railway-station the 

 track from which leads nowhere in particular, or a harbour on the top 

 of a hill. 



But it is to be particularly observed that the new formation does 

 not supplant the old, but works through and directs it. The neuro- 

 blasts of the cortex do not throw out their axons to make direct junc- 

 tion with muscles and sensory surfaces. Such junction the cortex 

 finds already established between the primitive cerebro-spinal axis and 

 the periphery. It joints itself on by nerve -fibres to the cells of the 

 central stem; and we have reason to believe that no single axon in an 

 ordinary spinal or cranial nerve* runs all the way from the periphery 

 to the cortex, and no axon of a cortical nerve-cell all the way from the 

 cortex to the periphery, but that the connection is made by a chain 

 of at least two neurons, the cell-body of one of which is situate in this 

 primitive grey tube. 



The fibres from the cortex of each cerebral hemisphere (corona 

 radiata), radiating out like a fan below the grey matter, are gathered 

 together into a compact leash as they sweep down through the isthmus 

 of the brain in the internal capsule, to join the crura cerebri. The 

 cortex of each cerebellar hemisphere, and the ribbed pouch of grey 



* The olfactory and possibly to some extent the optic nerves are exceptions 

 to this statement. Their relation to the cortex, as is easily understood from 

 the manner of their development (p. 822), is different from that of the other 

 nerves. 



