CHEMISTRY OF NERVE 795 



extract consisted of cholesterin, and 23-7 per cent, of kephalin; while 

 in the medullated fibres cholesterin made up only 25 per cent, of the 

 extract, and kephalin 12-4 per cent. On the other hand, the cere- 

 brosides are present, both relatively and absolutely, in much larger 

 quantity in medullated than in non-medullated nerves. In both 

 varieties of fibres kephalin, and not lecithin, is the chief phosphorus- 

 containing body (Falk). The medullary sheath further contains a 

 kind of network of a peculiar resistant substance, neurokeratin. The 

 neurilemma consists of substances insoluble in dilute sodium hydroxide. 

 Gelatin is obtained from the connective tissue which binds the nerve- 

 fibres together. There may also be ordinary fat in the meshes of the 

 epineurium connecting the bundles. Small quantities of xanthin, 

 hypoxanthin, and other extractives, can also be obtained from nerve. 

 According to Halliburton's analyses, the water in sciatic nerves amounts 

 to 65*1 per cent., and the solids to 34-9 per cent. The proteins make up 

 29 per cent, of the solids. 



For an analysis of the white matter of the brain, see Chapter XVI. 



Nerve-cells contain no potassium, according to Macallum; and this 

 is true both of the dendrites and the axons. In medullated nerves, how- 

 ever, potassium compounds are present external to the axons, chiefly at 

 the nodes of Ranvier (Frontispiece) and in the neurokeratin framework 

 of the sheath. 



The only chemical difference between living and dead nervous tissue 

 which has been made out with any degree of certainty is that the former 

 is neutral or faintly alkaline, and the latter acid, in reaction to such 

 indicators as litmus. This is especially true of the grey matter of the 

 central nervous system, although the white matter also is often found 

 acid. The change of reaction is due to the accumulation of lactic acid. 

 Such a change has not hitherto been clearly demonstrated in peripheral 

 nerves, either after death or after prolonged stimulation. The (non- 

 medullated) splenic nerves of the dog, even after stimulation for six 

 hours, never became acid (Halliburton and Brodie). 



Degeneration of Nerve. Nerve-fibres are ' bound in the bundle 

 of life ' with the nerve-cells from which their axis-cylinders arise; 

 the connection between cell and axon once severed, the nerve- 

 fibre dies inevitably. This is an illustration of a general law that 

 no portion of a cell can live once it is separated from the nucleus. 

 We shall see later on that changes also occur in the nerve-cell whose 

 axon has been divided from it, although they are of a different 

 nature (rather a slow atrophy than an acute degeneration), and 

 do not necessarily lead to the destruction of the cell. We must 

 regard the neuron not only as a morphological unit, a single cell 

 from nucleus to remotest end-brush, but also as a functional and 

 nutritive unit, the fortune of any portion of which is not in- 

 different to the rest. Thus, when a man's arm is amputated the 

 arm fares worse than the man, for the arm dies. But the man is 

 not unaffected. He lives, but he suffers much temporary disturb- 

 ance and some permanent loss. What is left of him is not quite 

 the same as it was. The acute changes that occur in severed nerve- 

 fibres are most conveniently studied in the peripheral nerves, 

 although essentially similar phenomena take place also in the fibres 

 of the central nervous system. 



