354 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



that portion of the central, organs which responds to the action of stimuli; or, 

 on the other hand, diminishing the normal excitability, to such a degree that 

 not only the ordinary stimuli, but excitants of unusual potency, occasion no re- 

 sponse. We shall hereafter see (CHAP. xiv. SECT. 7) that there is a very strong 

 probability, that a large proportion of the disordered actions of the Nervous 

 System depend upon the presence of poisonous matters in the blood, which have 

 not been introduced from without, but have been generated within the system 

 itself. 



358. All that we know respecting the conditions on which the production of 

 Nervous Force is dependent, supports the belief that its evolution involves a 

 change of composition in the Nervous matter; this change essentially consisting 

 in the cessation of its existence as a living tissue, and in the combination of 

 oxygen with its constituents; so that, in their restoration to the condition of 

 inorganic matter, the Vital force which was previously operative in the growth 

 and development of the tissue, is set free under this peculiar form. It may 

 serve to render this doctrine more intelligible, if we again refer to the analogy 

 of the Galvanic battery ; in which the change in the condition of the zinc (or 

 other oxidizable metal), which ceases to exist as such, and which enters into 

 new chemical combinations, is the condition of the evolution of the electric 

 force. The chief grounds for this doctrine will now be enumerated. 



359. In the first place, the dependence of Nervous energy upon the constant 

 circulation of Blood through the tissue, is much more close and immediate, 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 time for the elaboration of the nutrient materials that are to be withdrawn from 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 a reaction 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 nervous 

 activity. That the material supplied by the blood for this purpose is Oxygen, 

 would appear from a variety of considerations. A general survey of the Animal 

 kingdom shows, that oyxgen 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. 1 The effects of a sus- 

 pension of the oxygenating process (357) completely harmonize with these facts. 



360. Further, in proof that the activity of the Nervous system is immediately 

 dependent not upon a process of development or nutrition, but upon one of dis- 

 integration 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 interval of re- 

 pose, which we know to be favorable to the vegetative or reparative processes. 

 It is true that there are certain portions of it, particularly those that put in ac- 

 tion the respiratory muscles, which are in a state of unceasing though moderate 

 activity; and in these, the constant nutrition is sufficient to repair the effects of 

 the constant decay. But those parts which 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 reparation. Hence the sense of fatigue 

 which is experienced, when the Mind has been long acting through its instru- 

 ment, the Brain ; 3 and the irresistible tendency to sleep, which usually super- 



1 See "Princ. of Phys., Gen. and Comp., CHAP, xin., Am. Ed. 



2 Of the sense of fatigue induced by continued muscular exertion, it is difficult to say 

 how much is attributable to the state of the Muscles themselves ; it is better, therefore, to 

 base this statement upon cases in which the activity is purely Nervous. 



