PHYSIOLOGY. 



The motion of the blood in the minute 

 seems to be equal to its velocity in 

 the small arteries; this increases in the 

 larger trunks; and there is a constant ac- 

 celeration in the blood's course until it 

 arrives at the heart. This fluid is passing- 

 through tubes which constantly decrease 

 in area; and it follows of necessity, that 

 by diminishing the channel of a fluid, its 

 course must be accelerated. lie nee the 

 trunks of the ven cavx return to the 

 heart, within a given time, as much blood 

 as the aorta carried out of this viscus. 



The motion of the blood along the veins 

 must be derived from the impulse which 

 it receives from the heart, and from the 

 action (if there be any) of the arteries. 

 Its circulation in these vessels is aided by 

 the contraction of the muscles, which 

 must urge on the contained fluid towards 

 the heart; since their valves prevent any 

 retrograde motion. 



We shall readily perceive, that no cer- 

 tain calculation can be formed of the 

 powers of the heart, when we consider 

 that neither the quantity of blood ex- 

 pelled at one pulsation, nor the distance 

 through which it passes in a given time, 

 nor the velocity of its course, can be de- 

 jined with any certainty ; much less can 

 we form any accurate estimate of the ob- 

 stacles which occur to the blood's motion, 

 which must considerably affect such a 

 calculation. We may however approach 

 in some degree to the truth, by collecting 

 and comparing the results of probable 

 conjecture. If we calculate the blood 

 contained in the body at thirty pounds, 

 the number of pulsations in one minute 

 at seventy-five, and the quantity expelled 

 from the left ventricle at each pulsation 

 at two ounces and a half, the whole quan- 

 tity will pass through the heart about 

 twenty-two times in the course of an 

 hour ; and it will perform the circulation 

 once in less than three minutes. The 

 velocity with which this blood is pro- 

 pelled 'by the systole of the left ventricle 

 may be collected from the violence with 

 which it is ejected from a wounded 

 artery, and the altitude to which it as- 

 cends. Blumenbach has seen it projected 

 more than five feet from the carotid of an 

 adult during the first contractions of the 

 heart. Our countryman Hales calculated 

 from his experiments, in which he mea- 

 sured the height of the blood's ascent in 

 a glass tube, inserted into a large artery, 

 that it would be thrown seven feet and a 

 half from the human carotid: he estimates 

 the surface of the ventricle at fifteen, 

 square inches; and tiius finds that one 



thousand three hundred and fifty cubic 

 inches, or about fifty-one pounds weight, 

 press upon the left ventricle, and must 

 be overcome by its systole. Many other 

 calculations of the powers of the heart 

 have been formed upon mathematical 

 principles; but different persons have 

 been led to such opposite results, that we 

 are warranter! from this circumstance 

 in disregarding them altogether. Borelli 

 makes the powers of the heart equal to 

 an hundred and eighty thousand pounds; 

 Keill to eight ounces. Senac observes, 

 that if a weight of fifty pound be attached 

 to the foot, with the knee of that side 

 placed on the opposite one, the weight 

 will be elevated at each pulsation : this 

 weight is place<! at a considerable dis- 

 tance from the centre of motion ; and, 

 allowing for this circumstance, he esti- 

 mates the moving power at four hundred 

 pound. 



This power of the heart, so wonderful 

 both in extent and duration, must be re- 

 ferred to the irritability of the organ, in 

 which point of view it seems far to ex- 

 ceed all other muscular parts of the body. 

 That the immediate cause of contraction 

 in this viscus arises from the presence of 

 blood in its cavities, is shewn by the cele- 

 brated experiment of Haller; in which 

 the longer duration of action in the right 

 or left cavities, was varied by influencing 

 the supply of blood. 



In tlie action of those muscles that de- 

 pend on the will, a supply of nerves, and 

 a distribution of blood to the moving fi- 

 bres, seem to be essential conditions. It 

 has been disputed whether or not these 

 circumstances are necessary in the heart, 

 and what share they may contribute to 

 the heart's action. We may observe in 

 the first place, that the actions of the 

 heart are completely involuntary-, that 

 no exertion of the will can produce the 

 smallest effect in accelerating, retarding, 

 or otherwise affecting the actions of this 

 part. Yet various arguments prove that 

 the nerves exert an influence over this 

 organ. Not to mention the peculiar 

 arrangement of the cardiac nerves, the 

 sympathy between the heart's action, and 

 nearly every Other function, even of the 

 most different classes, suffices to demon- 

 strate the connection. The vehement 

 disturbance of the heart from the passions 

 of the mind, must be familiar to every 

 person from his own experience; its ac- 

 tion is also strongly influenced by various 

 states and affections of the alimentary 

 canal. 



The action of the heart is intimately 



