WASTE AND RENEWAL OF NERVOUS SUBSTANCE. 225 



No others of the soft tissues contain any large proportion of phospho- 

 rus ; and the marked increase in these deposits, which has been con- 

 tinually observed to accompany long-continued wear of mind, whether 

 by intellectual exertion, or by anxiety, can scarcely be set down to any 

 other cause. The most satisfactory proof is to be found in cases, in 

 which there is a periodical demand upon the mental powers; as, for 

 example, among clergymen, in the preparation for, and discharge of, 

 their Sunday duties. This is found to be almost invariably followed 

 by the appearance of a large quantity of the phosphates in the urine. 

 And in cases, in which constant and severe intellectual exertion has 

 impaired the nutrition of the brain, and has consequently weakened 

 the mental power, it is found that any premature attempt to renew the 

 activity of its exercise, causes the reappearance of the excessive phos- 

 phatic discharge, which indicates an undue waste of nervous matter. 



387. As the disintegration of the Nervous System is thus propor- 

 tional to its exercise, so must its reparation make a corresponding de- 

 mand upon the nutritive processes. And accordingly we find, that it 

 is very copiously supplied with blood-vessels ; arid that the amount of 

 food, appropriated to its maintenance in an active condition, is very 

 considerable. This we know from the fact, that persons of active minds, 

 but sedentary bodily habits, commonly require nearly as much food as 

 those, in whom the waste of the Muscular system is greater, and that 

 of the Nervous system less, in virtue of their bodily activity and the 

 less energetic operation of their minds. 



388. The nerve-fibres appear to originate, according to the observa- 

 tions of Prof. Kolliker, in cells which become fusiform by elongation 

 ( 193), and which then coalesce at their extremities ; and these seem 

 to increase, after the first formation of the trunks, by the longitu- 

 dinal subdivision of fusiform cells which had not previously undergone 

 complete metamorphosis into fibres, as well as by the development of 

 cells de novo. The nuclei of the original cells may be frequently seen 

 in the nerve-tubes at a later period, lying between their membranous 

 walls and the substance deposited in their interior. The earliest condi- 

 tion of both forms of nerve-fibre appears to be precisely the same ; but 

 the gelatinous remains in a state nearly resembling this^; whilst the 

 tubular is developed into a higher form. Various gradations, indeed, 

 may be traced between the two. The vesicular matter appears to be in 

 a state of continual change, as is the case with all cells whose functions 

 are active. The appearances observed by Henle, in the cortical sub- 

 stance of the brain lead to the belief, that there is as continual a suc- 

 cession of nerve-cells, as there is of epidermic cells ; their development 

 commencing at the surface, where they are most copiously supplied with 

 blood-vessels from the investing membrane, and proceeding as they^are 

 carried towards the inner layers, where they oome into more immediate 

 relation with the fibrous portion of the nerve-structure. This change 

 of place is probably due to the continual death and decay of the mature 

 cells, where they are connected with the fibres ; and the constant pro- 

 duction of new generations at the external surface, thus carrying t 

 previously-formed cells inwards, in precisely the same manner that 1 

 epidermic cells are progressively carried outwards. 



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