FUNCTIONS OF NERVOUS TISSUE. 353 



by pressure on the arterial trunks, by cold, or by any other cause), the check 

 is at once made apparent by the numbness of the surface; and a complete 

 stagnation produces complete insensibility. The power of receiving impressions 

 which excite reflex movements, is diminished in the same degree. 



356. On the other hand it is found, that increased circulation through the 

 same parts is attended with an exaltation of their function. This is particularly 

 noticed in those affections of the brain and spinal cord, closely bordering on in- 

 flammation, to which the terms active congestion and determination of blood have 

 been applied. We have, in such cases, extreme acuteness of sensation, excessive 

 activity of the mental functions, or violent excitement of the motor powers; accord- 

 ing (it would seem) to the particular division of the nervous centres most affected. 

 Again, we find that an increase in the circulation through any organ from 

 which afferent nerves arise, increases their readiness to receive impressions; thus 

 the sensibility of the genital organs of animals during the period of heat, and 

 of those of man in a state of venereal excitement, are greatly augmented ; and 

 the tendency of impressions made upon them to excite reflex movement, is simi- 

 larly exalted. 



357. The due activity of the Nervous System is not merely dependent upon 

 a constant and ample supply of Blood ; but it requires that this blood should 

 be in a state of extreme purity, and more especially that it should contain a 

 due supply of oxygen, and should be depurated of its carbonic acid, and of other 

 products of the decomposition of the body. The final cessation of nervous 

 power, in death by Asphyxia, is partly due (as will be shown hereafter, CHAP. 

 x. SECT. 3), to a positive deficiency in the supply of blood ; but the obtuseness 

 of sensibility which gradually increases until a state of unconsciousness comes 

 on, may be clearly traced in the first instance to the deficient aeration of the 

 blood, which is gradually deprived of its oxygen, and more and more charged 

 with carbonic acid. Corresponding but less severe 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. The retention of other excrementitious products in the 

 Blood is not less injurious, though its operation is less rapid. Thus, when the 

 elimination of Biliary matter is prevented, so that the blood becomes unduly 

 charged with its components, a great deficiency of Nervous power is manifested; 

 the general sensibility being rendered obtuse, the mental operations becoming 

 torpid, and the motor energy enfeebled ; and this state may become more and 

 more intense, with the increase of the accumulation, until, as in Asphyxia, the 

 entire functional activity of the Nervous system becomes extinct. The effect of 

 the retention of the materials of the Urinary excretion is not very dissimilar ; 

 but with the gradually deepening Coma, there are usually (as in Asphyxia) 

 convulsive movements. The influence of various poisons introduced into the 

 blood ab extra, upon the functional activity of the Nervous system, exhibits 

 hi a very marked manner the extreme importance of the purity of the circulat- 

 ing fluid, to the normal performance of the duties of this most important appa- 

 ratus. Thus we find the action of one class of poisons to commence with the 

 disturbance of the mental powers ; the control exercised by the will over the 

 course of thought is weakened, incoherence succeeds to regularity, passion takes 

 the place of calmness, and the state at last becomes one of maniacal delirium. 

 Another class acts primarily on the sensorial powers ; the consciousness of ex- 

 ternal impressions being first rendered obtuse, and then entirely destroyed; and 

 all the movements which are ordinarily excited or guided by it, being conse- 

 quently checked. And a third class operates especially upon the motor portion 

 of the nervous apparatus ; inducing an extraordinary degree of excitability in 



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