DEPENDENCE OF NERVOUS POWER ON SUPPLY OF BLOOD. 237 



on which its functional activity is dependent. The chief of these, is a con- 

 stant supply of oxygenated blood ; which is more necessary for the mainte- 

 nance of the Nervous power, than it is for that of any other tissue whatever. 

 This supply is peculiarly required at those points at which changes originate ; 

 not being, it would appear, so necessary for the mere conduction of impres- 

 sions. Consequently we find that the greatest supply of blood is afforded to 

 the nervous centres, and to the peripheral extremities or origins of the affe- 

 rent nerves ; and that the effects of any interruption to the supply are mani- 

 fested in an immediate and most striking manner. Thus, if the circulation 

 through the Brain be suspended but for an instant, insensibility and loss of 

 voluntary power supervene, and continue until it is restored. This was 

 shown by the following experiment of Sir A. Cooper's. After having tied 

 both carotid arteries in a dog, he compressed the Vertebral trunks ; and im- 

 mediate insensibility came on, the animal at the same time falling powerless. 

 But convulsive movements occurred at the same time ; showing that the func- 

 tions of the spinal cord were not suspended, but only deranged. As soon as 

 the blood was re-admitted to the brain, the animal recovered its consciousness 

 and voluntary power, and stood on its legs again ; the convulsive movements 

 ceased at the same time. In Syncope, the circulation through the Spinal 

 cord is weakened, by the failure of the heart's action, to the same extent as 

 the flow of blood through the Brain ; and a general cessation, not merely of 

 muscular movement, but of all power of exciting it, is the immediate result. 

 No sooner, however, is the circulation fully re-established, than the power of 

 the Nervous centres is restored. Again, the influence of diminished circula- 

 tion, at the origins of the afferent nerves, is shown in the deficient impressi- 

 bility of the nerves, at the part affected. Thus, if the movement of blood 

 through the capillaries of a limb be stagnated, whether by pressure on the 

 arterial trunks, by cold, or by any other cause, it is at once made apparent 

 by the numbness of the surface ; and a complete stagnation produces com- 

 plete insensibility. The power of receiving impressions, that are to excite 

 reflex movements, is diminished in the same degree. 



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

 same parts, is attended with an exaltation of their function. This is particu- 

 larly noticed in those affections of the brain and spinal cord, closely border- 

 ing on inflammation, 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 ; according (it would seem) to the particular division of the 

 nervous centres most affected. Again, we find that an increase in the circu- 

 lation 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 movements, is similarly exalted. 



292. 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 cessa- 

 tion of nervous power, in death by Asphyxia, is partly due (as will be shown 

 hereafter, Chap. XIII., 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 

 charged with more and more carbonic acid. Corresponding but less severe 



