THE CENTRAL NERVOUS SYSTEM 757 



neurons, it has been surmised that the cause of the alterations is 

 the loss of impulses which normally reach them along their dendrites. 

 In short, we may say with Marinesco, that the functional and 

 anatomical integrity of the neuron depends on the integrity of all 

 its constituent parts, and of the neurons which carry to it functional 

 excitations i.e., excitations connected with its proper physiological 

 work. ' The neuron, in fact, lives by its function, or, in common 

 language, by doing its work. Yet the anatomical tokens of mere 

 disuse, as in the motor cells of the anterior horn after division of the 

 cord at a higher level, are less distinct than those which follow 

 section of the axon. Therefore it must be concluded that the latter, 

 although not indispensable for the nutrition of the cell as the cell 

 is for the axon, exerts an influence upon it. Similar changes in the 

 chromatin may also be produced in nerve-cells by a period of 

 anaemia, in extensive superficial burns, in tetanus caused by the 

 injection of bacterial cultures, in acute alcoholic poisoning, in 

 fatigue, and in other ways. According to Wright, the inhalation 

 of ether or chloroform (in dogs) so alters the chromatic substance, 

 that it loses its affinity for aniline dyes. In long-continued anaes- 

 thesia the nucleus is also affected, while the nucleolus is the last 

 part of the cell to suffer. A greater alteration occurs in the cells 

 in the three hours between the sixth and ninth hours of anaesthesia 

 than in the five hours between the first and sixth. Although the 

 changes are transitory, the cells, after a narcosis of nine hours, being 

 practically normal in forty-eight hours, they indicate that the 

 duration of safe surgical anaesthesia has a limit measured by hours. 



It is probable that the alterations in the chromatic substance 

 should not be looked upon as the token of any specific lesion ; they 

 are the common structural response of the cell to injurious influences 

 of the most varied nature (p. 873). 



Grey and White Matter. Nerve-cells are the most distinctive 

 histological feature of the grey nervous substance. Sown thickly 

 in the cerebral cortex, the basal ganglia, the floor of the fourth 

 ventricle, and the cervical and lumbar enlargements of the cord, 

 they are scattered more sparingly wherever the grey matter extends. 

 They also occur in the spinal ganglia and their cerebral homologues 

 (such as the Gasserian ganglion), in the ganglia of the sympathetic 

 system, and the sporadic ganglia in general. But wide as is their 

 distribution, and great as is the size of the individual cells, some of 

 which have a diameter of 140 ^, or even more, they yet make up 

 but a small portion of the whole of the central nervous substance, 

 the total weight of the 9,000 millions of nerve-cell bodies in the 

 human brain being less than 27 grammes (Donaldson). And 

 although it is not to be wondered at that objects so notable when 

 viewed under the microscope should have struck the imagination of 

 physiologists, it is probable that the very high powers which it is 

 so common to attribute exclusively to them are, in part at least, 

 shared with the network or feltwork formed by their processes. 



The grey matter, in addition to this exceedingly delicate network 

 of non-medullated fibres and filaments representing the dendrites 

 and such axons and collaterals as terminate within itself, contains 

 also, as may be seen in preparations stained by Weigert's method,* 

 great numbers of exceedingly fine medullated fibres, many of which 

 are the collaterals of fibres that are passing out to the white matter. 



* Weigert's is a special method of staining the medullary sheath with 

 haematoxylin. 



