THE NERVOUS SYSTEM 297 



telegraph cable are enclosed. The sheath is of a phosphatic 

 fat, invested and held in place by a delicate transparent mem- 

 brane, neurilemma. Beneath this membrane nuclei occur at 

 regular intervals, and midway between each two nuclei the 

 sheath is cut across by a septum. Such interruptions or nodes 

 show that the sheath is not a part of the nerve, if the term is 

 used in the most restricted sense. Each internode is a cell 

 which has been wrapped round the nerve for its protection. 

 The axon with its sheath is spoken of as a nerve-fibre. A large 

 number of nerve-fibres bound together by connective tissue 

 constitute a nerve. In some cases the axon before it leaves the 

 spinal cord, but after it has entered its myelin sheath, gives off 

 one or two lateral branches (" collaterals "), which return to 

 arborize in the grey matter of the cord. It does not appear that 

 they are always present in the case of the motor neurones of the 

 spinal or cranial nerves probably they are usually omitted 

 but collaterals are important features of the large neurones of 

 the cortex of the cerebrum and cerebellum (Figs. 23, 24). Usually 

 two, three, or four such branches start off at right angles from 

 the axon, and after a time turn back towards the surface, 

 dividing into a few extremely slender branches. Their purpose 

 is an enigma. Possibly they bind a group of cells together in 

 functional unison. Such an explanation would seem reason- 

 able in the case of an arrangement of collaterals on the plan 

 we have just described ; but in various situations in the brain 

 cells are seen of which the axons, instead of becoming nerVe- 

 fibres, break up completely into collaterals, which branch 

 repeatedly. 



By various methods it may be shown that dendrites, cell- 

 body, and axon contain fibrils (Fig. 22). These neuro-fibrillse lie 

 parallel to one another in the axon. Where it divides they are 

 distributed amongst its branches. Possibly they also branch. 

 In the neurones of Malapterurus, already referred to, this would 

 appear to be inevitable. The discovery of neuro-fibrillae seemed 

 to carry us a step nearer to a comprehension of the physics 

 of nervous conduction. They clearly indicate that particles of 

 the substance of a nerve-fibre are oriented in the direction in 

 which impulses pass. It is a structural differentiation similar 

 to the fibrillation of muscle, and probably of the same order a 

 response to the same demand. But when we examine the 



