224 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



during the whole period of active life, is continually undergoing changes 

 in its substance, by decay and renewal. We know that, after death, it 

 is one of the first of all the animal tissues to exhibit signs of decompo- 

 sition ; and there is no reason to suppose, that this tendency is absent 

 during life. Hence for the simple maintenance of its normal character, 

 a considerable amount of nutritive change must be required. But many 

 circumstances further lead to the conclusion, that, like all other tissues 

 actively concerned in the vital operations, Nervous matter is subject 

 to. a waste or disintegration, which bears an exact proportion to the 

 activity of its operations ; or, in other words, that every act of the 

 Nervous system involves the death and decay of a certain amount of 

 Nervous matter, the replacement of which will be requisite in order to 

 maintain the system in a state fit for action. We shall hereafter see, 

 that there are certain parts of the Nervous system, particularly such as 

 put in action the respiratory muscles, which are in a state of unceasing, 

 though moderate, activity ; and in these, the constant nutrition is suffi- 

 cient to repair the effects of the constant decay. But those parts, which 

 operate in a more powerful and energetic manner, and which therefore 

 waste more rapidly when in action, need a season of rest for their repa- 

 ration. Thus a sense of fatigue is experienced, when the mind has been 

 long acting through its instrument the brain ; indicating the necessity 

 for rest and reparation. But when sleep, or cessation of the cerebral 

 functions, comes on, the process of nutrition takes place with unchecked 

 energy, counterbalances the results of the previous waste, and prepares 

 the organ for a renewal of its activity. In the healthy state of the 

 body, when the exertion of the nervous system by day does not exceed 

 that, which the repose of the night may compensate, it is maintained in 

 a condition which fits it for constant moderate exercise ; but unusual 

 demands upon its powers, whether by the long-continued and severe 

 exercise of the intellect, by excitement of the emotions, or by the com- 

 bination of both in that state of anxiety which the circumstances of 

 man's condition too frequently induce, occasion an unusual waste, 

 which requires, for the complete restoration of its powers, a prolonged 

 repose. 



385. There can be no doubt that (from causes which are not known), 

 the amount of sleep required by different persons, for the maintenance 

 of a healthy condition of the nervous system, varies considerably ; some 

 being able to dispense with it, to a degree which would be exceedingly 

 injurious to others of no greater mental activity. Where a prolonged 

 exertion of the mind has been made, and the natural tendency to sleep 

 has been habitually resisted, by a strong effort of the will, injurious 

 results are sure to follow. The bodily health breaks down, and too 

 frequently the mind itself is permanently enfeebled. It is obvious that 

 the nutrition of the Nervous system becomes completely deranged ; and 

 that the tissue is no longer formed, in the manner requisite for the dis- 

 charge of its healthy functions. 



386. As the amount of Muscular tissue that has undergone disinte- 

 gration, is represented (other things being equal), by the quantity of 

 urea in the urine, so do we find that an unusual waste of the nervous 

 matter is indicated by an increase in the amount of phosphatic deposits. 



