PHYSIOLOGY. 



a regular and continued succession to the 

 last period of life, and then all its parts 

 do not cease to act at once; but the 

 right auricle and ventricle survive the 

 opposite cavities for some little time, so 

 that the former part has been called the 

 ultimum moriens. The blood, which re- 

 turns by the venae cavx after the last ex- 

 piration, no longer finds the usual pas- 

 sage through the lungs, which are con- 

 tracted, but it is still urged on from be- 

 hind by that which the aorta has recently 

 propelled. Hence it is forced into the 

 right auricle, and excites contraction in 

 that part, by the stimulus of its presence, 

 some time after the left side has been at 

 rest. This congestion on the right side 

 of the heart in the last agony explains the 

 empty state of the arteries, particularly 

 the larger ones, after death. 



It is hardly possible to determine the 

 velocity of the blood's motion in the 

 healthy state ; for individuals differ from 

 each other in this respect, and considera- 

 ble variety probably takes place in diffe- 

 rent parts of the body. It is generally 

 supposed, that the blood flows in a more 

 gentle stream through the small arteries 

 than in the arterial trunks ; and that the 

 velocity of its current is somewhat less in 

 the veins than in the arteries of tlie body. 

 These differences have, however, been 

 exaggerated by former physiologists. The 

 mean velocity of the blood in the aorta is 

 calculated at eight inches for each pulsa- 

 tion, which gives about fifty feet in a 

 minute. If we reflect, that the systole of 

 the ventricle, which gives the whole im- 

 pulse to the blood, occupies only one- 

 third of the whole pulse, the velocity of 

 the blood's motion must be trebled in 

 that division of the time. It is said tliat 

 this velocity, which we have assigned to 

 the blood's motion at its departure from 

 the heart, becomes speedily diminished in 

 its further progress ; and the diminution 

 has been deduced from various causes. 

 The first and most powerful of these is 

 the constantly increasing area of the 

 branches, when compared with the trunk 

 of an artery. (See ANATOMY.) It is a 

 well-known law in hydraulics, that the 

 velocity of a fluid passing through an in- 

 verted cone constantly decreases, and 

 that the diminution of velocity is in the 

 ratio of the increase of area. The mathe- 

 matical physiologists have also noticed 

 the effects of friction ; deducing these 

 from a comparison with the course of 

 fluids in dead tubes. Other causes have 

 been derived from the same source ; hence 

 the serpentine course of some arteries, the 



unfavourable angles by which they some- 

 times arise, and their communications with 

 each other, are enumerated among ihe cir- 

 cumstances which retard the course of the 

 arterial blood. But it must be remember- 

 ed, that in viewing these retarding cau- 

 ses we are considering their action on 

 the blood, as if this fluid were contained 

 in inanimate tubes, and influenced merely 

 by the contraction of the heart, without 

 taking into the account any accessory 

 impulse, which may be, and probably is, 

 derived from the arteries. This retarda- 

 tion has been variously estimated by dif- 

 ferent calculators, who have all made it 

 very considerable. Hales supposes the 

 blood to flow through the capillary arte- 

 ries of a frog at the rate of two-thirds 

 of an inch in a minute, which will be 

 about 650 times slower than in the human 

 aorta. Robinson and Whytt have gone 

 still further : the former stating, that the 

 velocity of the blood's motion in the aor- 

 ta is to that in the smallest vessels as 

 1100 to 1. We mention these calcula- 

 tions, to shew what absurdities have been 

 committed by men of the greatest abili- 

 ties, when they have applied the Ja\rs 

 which regulate the properties of dead, 

 matter to the living functions of the ani- 

 mal machine. Haller's observations on 

 the circulation in living animals, (Elem. 

 Phys. lib. vi. sect. 1. . 30) entirely over- 

 throw these calculations. He found by his 

 microscopical experiments, that the blood 

 flowed generally as rapidly through the 

 small as through the larger vessels. He 

 states also, that in living animals it is 

 poured out as far from a small as from a 

 large artery. The numerous and diver- 

 sified experiments of Spallanzani afford 

 additional evidence of the same truth. 



We have stated, that the blood is 

 thrown into the arteries by separate con- 

 tractions of the heart ; yet these vessels 

 are constantly full, as may be proved by 

 opening them during the heart's diastole. 

 For the blood flows on in such a way, 

 that the subsequent quantity discharged 

 from the right ventricle, overtakes that 

 which is before, and thus causes the pul- 

 sation of the arteries. Tlie excess of ve- 

 locity in the blood coming from the heart, 

 over that contained in the arteries, be- 

 comes constantly less ; and at a certain 

 point ceases altogether. Here the pulse 

 ceases also : hence in microscopical ob- 

 servations on the course of the blood in 

 small vessels, its stream appears to be 

 uniform; and it is commonly stated, that 

 the pulsation ceases in vessels of about 

 one sixth of a line in diameter. 



