238 FUNCTIONS OF THE NERVOUS SYSTEM. 



symptoms occur, when the excretion of carbonic acid is not checked, but only 

 slightly impeded ; provided the impediment be in operation for a sufficient 

 length of time, as in the case of an ill-ventilated apartment ; an indisposition 

 to mental exertion, a deficiency of muscular power, and an obtuseness of the 

 intellectual and moral faculties, being the general result. These facts are 

 readily explained upon the hypothesis (which seems now to have a sufficiently 

 wide foundation, to be entitled to rank as physiological truth, although no 

 very direct proof of it can be given), that the functional activity of the nerv- 

 ous system is mainly dependent upon the combination of the oxygen sup- 

 plied by the blood, with its elements; the production of the nervous force, 

 whatever be its nature, being a result of this change of composition. The 

 chief grounds for this doctrine will now be enumerated. 



293. In the first place, the dependence of nervous energy upon the con- 

 stant circulation of blood through the tissue, is much more close and imme- 

 diate than can be accounted for on the idea that the relation is one of mere 

 nutrition or development. On the contrary, where these last changes are 

 taking place most actively, we often find rather a disposition to stagnation of 

 the current, to give time for the elaboration of the nutrient materials that are 

 to be withdrawn from it ; and in no case does the process so instantaneously 

 cease, when the flow is suspended. From this it would appear, that some 

 combination takes place between the elements of the nervous tissue, and some 

 material supplied by the blood ; which is much more rapid in its character, 

 than the process of cell-development ; and which is essentially concerned in 

 the production and maintenance of the active condition of the nervous sys- 

 tem. Again, that the material supplied by the blood for this purpose is Oxy- 

 gen, would appear from a variety of considerations. A general survey of 

 the Animal kingdom shows, that oxygen, is essential to the maintenance of 

 animal life, as distinct from vegetative; and a more particular comparison 

 of different tribes demonstrates most unequivocally, that the consumption of 

 oxygen is in direct relation to the development of the animal powers in each. 

 These facts harmonize completely with what has been just stated, respecting 

 the effects of a suspension of the oxygenating process. 



294. Further, in proof that the activity of the Nervous system is immedi- 

 ately dependent, not upon a process of development or nutrition, but upon 

 one of disintegration or destruction, it may be urged, 'that it is impossible for 

 this state of activity to be maintained, in a large portion of it, without an in- 

 terval of repose, which we know to be favourable to the vegetative or repa- 

 rative processes. There are certain parts of the Nervous System, particularly 

 those that put in action the respiratory muscles, which are in a state of un- 

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

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

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

 more rapidly disintegrated when in action, need a season of rest for their re- 

 paration. Hence the sense of fatigue which is experienced when the mind 

 has been long acting through its instrument, the Brain ; and the irresistible 

 tendency to sleep, which usually supervenes after any unusual exertion of this 

 kind. In the healthy state of the body, when the exercise of the nervous system 

 by day does not exceed that which the repose of the night may compensate, 

 the Nervous System is maintained in a condition which fits it for constant 

 moderate exercise ; but unusual demands upon its powers, whether by long- 

 continued and severe exercise of the intellect, by excitement of the emotions, 

 or by the combination of both, in that state of anxiety which the circum- 

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

 and require a prolonged repose and uninterrupted nutrition, for the complete 

 restoration of its powers. There can be no doubt that (from causes which 



