230 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



nervous force is essentially concerned in this operation. But, on the 

 other hand, it is quite certain that the influence transmitted along the 

 nerves of the living body is not ordinary electricity ; for all attempts 

 to procure manifestations of electric changes in the state of nerves, 

 that are acting most energetically on muscles, have completely failed ; 

 and a nerve remains capable of conveying the influence of electricity, 

 when it has ' been rendered unable to transmit the influence of the 

 brain, as by tying a ligature round it, or by tightly compressing it 

 between the forceps, which gives no interruption to the one agency, 

 while it completely checks the other. Notwithstanding, then, the strong 

 analogy which exists between these two powers, we are not warranted 

 in regarding them as identical; but they have towards each other that 

 relation of reciprocity, which exists between Electricity and Heat, or 

 between Electricity and Magnetism, each being convertible into the 

 other in a certain definite ratio ( 53). 



397. It is more desirable, however, that we should understand the 

 conditions under which the phenomena of the Nervous System take 

 place, than that we should spend much time in discussing the identity 

 of its peculiar powers with any others in Nature. The conducting 

 power of the nervous fibres appears to remain with little decrease for 

 some time after death, especially in cold-blooded animals ; for we can, 

 by pinching, pricking, or otherwise stimulating the motor-trunk, give 

 rise to contractions in the muscles supplied by them, exactly as during 

 life. This power is much lessened by the influence of narcotics ; so that 

 if a nervous trunk be soaked in a solution of opium, belladonna, or other 

 powerful narcotic, it ceases to be able to convey the effects of stimuli to 

 the muscles, some time before the muscles themselves lose their con- 

 tractile power. On the other hand, it seems to be exalted by various 

 irritating influences ; so that, when the nervous trunk has been treated 

 with strychnia, or when it has been subjected to undue excitement in other 

 ways, a very slight change is magnified (as it were) during its transmis- 

 sion, and produces effects of unusual intensity. 



398. Now although the conducting power of the fibrous structure 

 will continue for a time, after the circulation through it has ceased, the 

 peculiar endowments of the vesicular substance, by which it originates 

 the changes which the former transmits, are only manifested, when 

 blood is moving through its capillaries. Thus if the circulation through 

 the brain cease but for a moment, total insensibility, and loss of the power 

 of voluntary motion, immediately supervene. The brain is supplied 

 with blood through four arteries, the two internal carotids, and the 

 two vertebrals ; and by the communication of these with each other 

 through the circle of Willis, the circulation will still be kept up, if only 

 one of them should convey blood into the cavity of the cranium. Hence 

 it is necessary that the flow of blood should be checked through all of 

 them, in order that the functions of the brain should be suspended ; and 

 the suspension is then complete and instantaneous. The best method 

 of effecting this was devised by Sir Astley Cooper. He tied both the 

 carotid arteries in a dog : which, for the reasons just mentioned, did 

 not produce any decided influence on the functions of the brain, the 

 circulation being kept up through the vertebrals. But upon compress- 



