660 CIRCULATION. 



iind are exceedingly unsatisfactory. Haller, in the body may be estimated at nearly thirty 



who fully admitted the greater capacity of the pounds : now, let us suppose the aorta and 



smaller arteries, arid allowed that the flow of pulmonary arteries, together with their return- 



the blood must therefore, from hydraulic prin- ing veins, to form a continuous tube of the 



ciples, become less rapid in passing from the length of the two courses of the blood, in the 



trunks to their branches, a proposition which systemic and pulmonic circulations, and of the 



he illustrates by comparing the stream of blood same diameter as these vessels at their point of 



in its passage to a river which enters a lake, junction with the heart; a very simple calcula- 



was yet inclined, from the result of his actual tion shews us that such a tube is capable of 



observations, to deny that the velocity is much holding only about six pounds and a quarter, 



less in the smaller than in the larger arteries, or less than a fourth part of the whole blood 



Spallanzani, although admitting more explicitly of the body ; or in other words, were the aggre- 



still than Haller the necessity of such a retarda- gate capacities of the small vessels no more 



tion, seems to have met with the same difficulty than equal to that of the larger, they would be 



in reconciling theory with his attempts to mea- capable of holding only a fifth of the blood 



sure the velocity of the blood in the small ves- contained in the body. 



sels : and both these authors state, that although The velocity of the blood in the commence- 



the circulation was in general comparatively ment of the aorta may be considered as two 



slow in the web of the frog's foot, still in many and a half inches in a second, for this is the 



instances in this situation, and more frequently space occupied by all the blood which is pro- 



in the mesenteiy, they were unable to detect pelled into the aorta from the left ventricle in 



any difference in the rapidity of the flow of the that time, and according to the arbitrary modes 



blood in the larger and smaller arteries.* of estimating the relative capacity of the aorta 



Hales, again, states as the result of his ob- and its branches here employed, the velocity 



servations and measurements, that the velocity of the blood in the aortic capillaries generally, 



of the blood in the smallest capillaries of the might be considered as one-fourth of that in the 



abdominal muscles of the frog, is so small as commencement of the aorta, or nearly half an 



one or one and a half inch in a minute; and, inch in a second, a result widely different from 



from the attempts which we ourselves have that obtained by Hales. 



made at these measurements, we feel inclined Attempts have also been made to estimate 



to agree with the statement of this able experi- the velocity of the flow of blood, by ob- 



menter, having, upon several occasions, ascer- serving the time which certain substances, 



tained that in those capillaries which admit when introduced into one part of the vascular 



only two globules of blood, the velocity is not system, take to pass to another. The most 



greater than the hundredth part of an inch in a remarkable series of experiments of this na- 



second ; but it seems doubtful whether in all ture with which we are acquainted were per- 



the capillaries the velocity is so small as in formed by Hering.* This author states that 



those just alluded to, and in the larger capillary he has been able to detect prussiate of potassa, 



vessels of the diameter of six globules, when which he had introduced into one of the jugu- 



no unnatural obstruction to the circulation in lar veins of a horse, in the blood drawn from 



the limb occurred, independently of the diffi- the opposite jugular vein in the space of from 



culty of fixing the eye upon any globule in such twenty to thirty seconds ; and he has formed 



a way as to trace its progress along the vessel, the conclusion from this experiment that the 



the velocity has always appeared so great as to prussiate of potass, in order to gain the jugu- 



prevent the possibility of measuring it; and we lar vein on the opposite side of the body, had 



are at a loss to conceive in what manner Haller passed in this remarkably short space of time 



made the comparison he speaks of between the through the whole course of the double cireu- 



velocity in the larger and smaller arteries. By lation : that it was first carried to the heart, 



means of the microscope, it is easy to see that then passed through the pulmonary arteries 



the velocity is greater in the small arteries than and veins, and returned to the heart, from 



in the corresponding veins, which are both which it must have been transmitted through 



more numerous and considerably larger than the ultimate ramifications of the systemic ar- 



the arteries. teries before being brought back by the veins, 



The results of actual observation of the flow in which it was found on the opposite side of 

 of the blood and of the measurement of the the body. Hering states, as the result of other 

 relative capacities of different arteries, afford as experiments of a similar nature made upon 

 yet very unsatisfactory data upon which to different bloodvessels, that the prussiate of 

 found an estimate of the relative velocity of the potassa passed from the jugular vein to the 

 blood in the trunks and branches of the arte- saphena vein in twenty seconds ; to the mas- 

 ries. In the absence of more direct means of seteric artery, in fifteen to twenty seconds ; to 

 calculation, an approximative estimate may be the external maxillary artery, in ten to twenty- 

 made in another way, viz. by comparing the five seconds; to the metatarsal artery, in twenty 

 quantity of blood which occupies a known to forty seconds. 



space of the larger vessels with the whole quan- We consider these curious experiments as 



tity of blood contained in the body. important in many points of view, but do not 



We have already seen that the whole blood feel inclined to concur in the conclusion de- 

 duced from them by their author, that tiie 



* Haller appears to mean here arteries of consi- 

 derable size. * Tiedcmann's Zeitscluiit, vol. in. p. bo. 



