147 



Those who admit, to its full extent, Harvey's opinion on the circula- 

 tion of the blood, and who think with him, that the heart is the sole agent 

 of the circulation, overrate the power of that organ, so as to proportion 

 it to the extent of the course which the blood is to take, and to the num- 

 ber of the obstacles which it is to meet in its way. But, as I am about 

 to state, the blood-vessels should not be considered as inert tubes, in 

 which the blood flows from the mere impulse which it has received from 

 the heart. 



LIV. Of the action of the arteries. There is no part of the body to which 

 the .heart does not send blood by the arteries, for, it is impossible to 

 make a puncture, with the finest netdle, into any of our organs, without 

 wounding several of these vessels, and causing an effusion of blood. The 

 aortic arterial system may be compared to a tree, whose trunk, repre- 

 sented by the aorta, having its root in the left ventricle of the heart, ex- 

 tends afar its branches, and throws out, on every side, its numerous ra- 

 mifications. The siz3 of the arteries decreases, the farther they are from 

 the trunk by which they are given off. Their form, however, is not that 

 ot a cone, they are rather cylinders arising from one another, and de- 

 creasing successively in size. As the branches given off by a trunk, taken 

 collectively, have a greater diameter than that of the trunk itself, the ca- 

 pacity of the arterial system increases with the distance from the heart, 

 hence it follows, that as the blood is continually flowing from a straighter 

 to a wider channel, its course must slacken. The direction of the arte- 



2. The degree of velocity with which it is expelled. 



3. The amount of resistence which each ventricle lias to overcome, before it can 

 propel the blood into its corresponding arteries. 



4. The effects of the action of" the heart on the blood. 



But these are points which seem likely never to be ascertained with any sort of pre- 

 cision, and of course Aur computations must continue as heretofore, vague and conjec- 

 tural. All we know with certainty on the subject is, that the heart is a muscle of great 

 strength, as is evinced by the phenomena of the circulation, and further, by the fact 

 that, if the heart of a living animal be grasped, no effort of the hand will repress its 

 action. 



An inquiry far more interesting here presents itself. Why, as has been frequently 

 asked, does not the heart become exhausted, like other muscles, by exertion ? 



Three answers to this intricate question have been attempted, no one of which, how- 

 ever, is at all satisfactory. 



1. By Willis it is maintained, that the voluntary muscles derive their nerves from the 

 cerebrum, while the cerebellum supplies the heart and other involuntary muscles ; and 

 hence he infers that the one set is thereby fitted for temporary, and the other for per- 

 manent, and uninterrupted action. 



Admitting the statement to be anatomically correct, we do not perceive that it leads 

 to any such conclusion. But it is not so. There are many exceptions to this alleged dis- 

 tribution of the nerves. 



2. By Stahl, it is imputed altogether to the intelligence of his anima medica, a guar- 

 dian power, that foreseeing the danger to which the system would be exposed by any 

 remission in the action of the heart, ordains it otherwise. This is all hypothesis, and 

 of the most wretched kind too ; by the adoption of which we only cut the knot that 

 perplexed us to untie. 



3. By Haller it is accounted for on the supposition of a larger share of irritability be- 

 ing possessed by the heart. It is to be recollected, that he judged of the degree of 

 this property by the duration of the contractile power in a part, after the death of the 

 animal. Taking this as a just criterion, it will appear from the experiments of Fowler, 

 that in cold-blooded animals, at least, the voluntary museles retain their irritability longer 

 than the heart itself. The difference, indeed, in this respect, in any class of animals, 

 is so slight, that other objections aside, the hypothesis could not be entertained. 

 Chapman. 



