NERVE 691 



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 p. 88 1. 



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

 this is true both of the dendrites and the axons. In medullated 

 nerves, however, 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 destruc- 

 tion 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 indifferent 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 disturbance 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. 



A spinal nerve is composed of efferent fibres whose cells of 

 origin are in the grey matter of the anterior horn, and afferent 

 fibres whose cells of origin are in the posterior root ganglion. 

 When such a nerve is cut below the junction of its roots, muscular 

 paralysis and impairment of sensation at once follow in the 



442 



