672 A MANUAL OF PHYSIOLOGY 



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 accom- 

 panied with a corresponding development of nerve-fibres, for an 

 isolated nerve-cell 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 

 junction with muscles and sensory surfaces. Such junction the cor- 

 tex 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 linking cortex with cortex, or central 

 ganglia with each other. In the third group are included 

 (a) fibres which connect portions of the cortex on the same 

 side (association fibres) and (6) fibres which connect por- 



* 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. 660), is different from 

 that of the other nerves. 



