660 



CIRCULATION. 



and are exceedingly unsatisfactory. Haller, 

 who fully admitted the greater capacity of the 

 smaller arteries, and allowed that the flow of 

 the blood must therefore, from hydraulic prin- 

 ciples, become less rapid in passing from the 

 trunks to their branches, a proposition which 

 he illustrates by comparing the stream of blood 

 in its passage to a river which enters a lake, 

 was yet inclined, from the result of his actual 

 observations, to deny that the velocity is much 

 less in the smaller than in the larger arteries. 

 Spallanzani, although admitting more explicitly 

 still than Haller the necessity of such a retarda- 

 tion, seems to have met with the same difficulty 

 in reconciling theory with his attempts to mea- 

 sure the velocity of the blood in the small ves- 

 sels : and both these authors state, that although 

 the circulation was in general comparatively 

 slow in the web of the frog's foot, still in many 

 instances in this situation, and more frequently 

 in the mesentery, they were unable to detect 

 any difference in the rapidity of the flow of the 

 blood in the larger and smaller arteries.* 



Hales, again, states as the result of his ob- 

 servations and measurements, that the velocity 

 of the blood in the smallest capillaries of the 

 abdominal muscles of the frog, is so small as 

 one or one and a half inch in a minute; and, 

 from the attempts which we ourselves have 

 made at these measurements, we feel inclined 

 to agree with the statement of this able experi- 

 menter, having, upon several occasions, ascer- 

 tained that in those capillaries which admit 

 only two globules of blood, the velocity is not 

 greater than the hundredth part of an inch in a 

 second ; but it seems doubtful whether in all 

 the capillaries the velocity is so small as in 

 those just alluded to, and in the larger capillary 

 vessels of the diameter of six globules, when 

 no unnatural obstruction to the circulation in 

 the limb occurred, independently of the diffi- 

 culty of fixing the eye upon any globule in such 

 a way as to trace its progress along the vessel, 

 the velocity has always appeared so great as to 

 prevent the possibility of measuring it ; and we 

 are at a loss to conceive in what manner Haller 

 made the comparison he speaks of between the 

 velocity in the larger and smaller arteries. By 

 means of the microscope, it is easy to see that 

 the velocity is greater in the small arteries than 

 in the corresponding veins, which are both 

 more numerous and considerably larger than 

 the arteries. 



The results of actual observation of the flow 

 of the blood and of the measurement of the 

 relative capacities of different arteries, afford as 

 yet very unsatisfactory data upon which to 

 found an estimate of the relative velocity of the 

 blood in the trunks and branches of the arte- 

 ries. In the absence of more direct means of 

 calculation, an approximative estimate may be 

 made in another way, viz. by comparing the 

 quantity of blood which occupies a known 

 space of the larger vessels with the whole quan- 

 tity of blood contained in the body. 



We have already seen that the whole blood 



* Haller appears to rm-an here arteries of consi- 

 derable size. 



in the body may be estimated at nearly thirty 

 pounds : now, let us suppose the aorta and 

 pulmonary arteries, together with their return- 

 ing veins, to form a continuous tube of the 

 length of the two courses of the blood, in the 

 systemic and pulmonic circulations, and of the 

 same diameter as these vessels at their point of 

 junction with the heart; a very simple calcula- 

 tion shews us that such a tube is capable of 

 holding only about six pounds and a quarter, 

 or less than a fourth part of the whole blood 

 of the body ; or in other words, were the aggre- 

 gate capacities of the small vessels no more 

 than equal to that of the larger, they would be 

 capable of holding only a fifth of the blood 

 contained in the body. 



The velocity of the blood in the commence- 

 ment of the aorta may be considered as two 

 and a half inches in a second, for this is the 

 space occupied by all the blood which is pro- 

 pelled into the aorta from the left ventricle in 

 that time, and according to the arbitrary modes 

 of estimating the relative capacity of the aorta 

 and its branches here employed, the velocity 

 of the blood in the aortic capillaries generally, 

 might be considered as one-fourth of that in the 

 commencement of the aorta, or nearly half an 

 inch in a second, a result widely different from 

 that obtained by Hales. 



Attempts have also been made to estimate 

 the velocity of the flow of blood', by ob- 

 serving the time which certain substances, 

 when introduced into one part of the vascular 

 system, take to pass to another. The most 

 remarkable series of experiments of this na- 

 ture with which we are acquainted were per- 

 formed by Hering.* This author states that 

 he has been able to detect prussiate of potassa, 

 which he had introduced into one of the jugu- 

 lar veins of a horse, in the blood drawn from 

 the opposite jugular vein in the space of from 

 twenty to thirty seconds ; and he has formed 

 the conclusion from this experiment that the 

 prussiate of potass, in order to gain the jugu- 

 lar vein on the opposite side of the body, had 

 passed in this remarkably short space of time 

 through the whole course of the double circu- 

 lation : that it was first carried to the heart, 

 then passed through the pulmonary arteries 

 and veins, and returned to the heart, from 

 which it must have been transmitted through 

 the ultimate ramifications of the systemic ar- 

 teries before being brought back by the veins, 

 in which it was found on the opposite side of 

 the body. Hering states, as the result of other 

 experiments of a similar nature made upon 

 different bloodvessels, that the prussiate of 

 potassa passed from the jugular vein to the 

 saphena vein in twenty seconds ; to the mas- 

 seteric artery, in fifteen to twenty seconds ; to 

 the external maxillary artery, in ten to twenty- 

 five seconds ; to the metatarsal artery, in twenty 

 to forty seconds. 



We consider these curious experiments as 

 important in many points of view, but do not 

 feel inclined to concur in the conclusion de- 

 duced from them by their author, that the 



' Ticdcmunn's Zciischrift, vol. iii. p. 85. 



