232 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



its powers is the result, their action being precisely that of narcotic 

 poisons. Various substances introduced into the blood may exert similar 

 influences ; depressing the activity of the vesicular substance of the 

 nervous centres, and consequently producing torpidity, not merely in 

 regard to the reception of impressions, and the performance of volun- 

 tary motions, but also in the mental operations generally. 



400. On the other hand, various conditions of the blood, especially 

 those depending on the presence of certain external agents, produce an 

 undue energy in the functions of the nervous centres ; which energy, 

 however, is almost invariably accompanied by irregularity, or want of 

 balance among the different actions. Of this we have a familiar example 

 in the operation of alcohol. Its first effect, when taken in moderate 

 quantity, is usually to produce a simple increase in the activity of the 

 cerebral functions. A further dose, however, occasions not merely an 

 increase, but an irregularity ; destroying that power of self-control, which 

 is so important a means of balancing the different tendencies in the 

 healthy condition of the mind. And a still larger dose has the effect 

 of a narcotic poison ; producing diminution or suspension of activity in 

 all the functions of the brain. In some persons, this is the mode in 

 which the alcohol acts from the first, its stimulating effects being altogether 

 wanting. A similar activity is usually produced by the respiration of 

 the Nitrous Oxide, which seems to increase all the powers of the mind, 

 save that of self-control, which it diminishes ; the individual, while under 

 its influence, being the slave of his impulses, which act on his muscular 

 system with astonishing energy. Very analogous to this, is the incipient 

 stage of mania ; which is simply an undue energy of the cerebral func- 

 tions at first in some degree under the control of the will, but after- 

 wards increasing to an extent that renders the individual completely 

 powerless over himself; and showing itself in the intensity of the sensa- 

 tions produced by external objects, in the vividness of the trains of 

 thought (which, being entirely uncontrolled, succeed each other with 

 apparent irregularity, though probably according to the laws of associa- 

 tion and suggestion), and in the violence of the muscular actions. Such 

 a state may continue for some time, without the intervention of sleep ; 

 but the subsequent exhaustion of nervous power is proportioned to the 

 duration of the excitement; and frequent attacks of mania almost 

 invariably subside at last into imbecility. 



401. In these cases of undue excitement, there is obviously an increase 

 in the supply of blood to the head ; as indicated by the suffusion of the 

 face, the injection of the conjunctiva, the throbbing at the temples, the 

 pulsation of the carotids ; and we find that measures which diminish 

 the activity of the circulation through the brain, are those most effec- 

 tual in subduing the excitement. But it does not at all follow, that 

 this undue action of the brain should be connected with an excess in the 

 whole amount of nutritive material, and should require general deple- 

 tion for its treatment. In fact, a very similar class of symptoms may 

 present itself under two conditions of an entirely opposite kind, 

 inflammation, accompanied with an increase in the proportion of fibrine 

 in the blood, and requiring treatment of a lowering kind, and irritation, 

 depending on a state of blood in which there is a deficiency of solid 



