THE CIRCULATION OP THE BLOOD. 223 



only one open to it into the pulmonary artery, and by its 

 branches is conveyed to the capillaries of the lungs, from which 

 it is returned freed from mu^h of its carbonic anhydride and 

 replenished with oxygen, to the left auricle, whence it proceeds 

 in a similar manner into the great arterial main, the aorta, for 

 general distribution throughout the smaller arteries and the 

 capillaries to the most remote as well as the nearest parts, from 

 which it is gathered up and returned laden with many impuri- 

 ties, and robbed of a large proportion of its useful matters, to 

 the right side of the heart. 



It will be remembered that corresponding subdivisions of 

 each side of the heart act simultaneously, and that any decided 

 departure from this harmony of rhythm would lead to serious 

 disturbance. 



THE VELOOITT OF THE BLOOD AND BLOOD-PRESSTTRE. 



If the relative capacity and arrangement of the various parts 

 of the circulatory system be as has been represented, it follows 

 that we may predict with some confidence, apart from experi- 

 ment, what the speed of the ilow and the vascular tension must 

 be in different parts of the course of the circulation. 



We should suppose that, in the nature of the case, the veloc- 

 ity would be greatest in the large arteries, gradually diminish 

 to the capillaries, in which it would be much the slowest and, 

 getting by degrees faster, would reach a speed in the largest 

 veins approaching that of the corresponding arteries. 



The methods of determining the velocity of the blood-stream 

 have not entirely surmounted the diflQculties, but they do give 

 results in harmony with the above-noted anticipations. 



The area of the great aortic trunk being so much less than 

 that of the capillaries, the How in that vessel we should expect 

 to be very much swifter than in the arterioles or the capillaries. 

 Moreover there must be a great difference in the velocity during 

 cardiac systole and diastole, and according as the beat of the 

 heart is forcible or otherwise. But apart from these more ob- 

 vious differences, there are variations depending on complex 

 changes in the peripheral circulation, owing to the frequent 

 variations in the diameter of the arterioles in different parts, 

 as well as differences in the resistance offered by the capillaries, 

 the causes of which are but ill understood, though less obscure, 

 we think, than they are often represented to be. Since for the 



