760 A MANUAL OF PHYSIOLOGY 



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

 cerebral vesicles I. and III. (Fig. 299) (the grey matter of the cerebral 

 and cerebellar cortex) comes to overshadow the superficial grey 

 matter hitherto present only in the roof of vesicle II. (in the corpora 

 bigemina). And this cortex formation becomes larger in amount, 

 and, in the case of the cerebral grey matter, more richly convo- 

 luted, 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 sub- 

 stance 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 complete develop- 

 ment) 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 

 neuroblasts of the cortex do not throw out their axons to make 

 direct junction with muscles and sensory surfaces. Such junction 

 the cortex finds already established between the primitive cerebro- 

 spinal axis and the periphery. It joins 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 matter, known as the corpus dentatum, which is buried in 

 its white core, are also connected by strands of fibres with the 

 central stem and the cerebral mantle. The restiform body or 

 inferior peduncle brings the cerebellum into communication with the 

 spinal cord. The superior peduncle by one path, and the middle 

 peduncle by another, connect it with the cerebral cortex. A great 

 transverse commissure, the corpus callosum, unites the cerebral 

 hemispheres across the middle line, while transverse fibres that 

 break through the middle lobe or worm, form a similar though 

 far less massive junction between the two hemispheres of the 

 cerebellum. 



The fibres of the nervous system may be divided into (i) fibres 

 connecting the peripheral organs with nerve-cells in the central 

 grey axis ; (2) fibres connecting nerve-cells in this central axis 

 with cells in the external or cortical grey tube ; and (3) fibres 



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

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

 understood from the manner of their development (p. 747), is different 

 from that of the other nerves. 



