CIRCULATION. 



659 



refer the phenomena of the circulation, as fat- 

 as we can, to hydraulic principles, which, when 

 correctly applied, must form the only sure 

 guide in conducting a physiological inquiry of 

 this nature. 



The flow of the blood, as it is expelled from 

 the left ventricle, may be said to be intermit- 

 tent, for it moves only at the time of the ventri- 

 cular systole. Farther on in its course, in the 

 larger as well as the middle sized arteries, the 

 flow of blood is remittent, or is more rapid 

 after each beat of the heart, and by the time it 

 arrives at the capillary vessels and commence- 

 ment of the veins, the velocity is rendered per- 

 fectly uniform. The effect, therefore, produced 

 by the arterial tubes is to convert an intermittent, 

 first into a remittent, and afterwards into a uni- 

 form force. When an opening is made into 

 one of the larger arteries, the jet of blood which 

 issues is regularly increased in velocity at every 

 systole of the ventricle. In the very small ar- 

 teries, this acceleration of the stream becomes 

 less perceptible. We know that it has altoge- 

 ther disappeared in the smallest vessels or ca- 

 pillaries, from microscopic observation of the 

 flow of the blood in them, and the uniformity 

 of the velocity of the stream in the veins is 

 clearly shewn in all instances in which a vein 

 is opened, as in the common operation of 

 bleeding from the arm. 



Various circumstances shew that in the living 

 body the blood forms an uninterrupted column 

 of fluid in the bloodvessels, and that the whole 

 vascular system is kept in a state of forced dis- 

 tension by the reiterated impulses communi- 

 cated to the blood by the ventricular contrac- 

 tions. Besides the general fulness of the blood- 

 vessels and their connection with the heart, we 

 may mention as proofs of the distended state of 

 the vascular system, the facts, 1st, that, on 

 opening any of the bloodvessels, the blood 

 issues with greater force at the first moment 

 than afterwards ; and 2d, that when we imitate 

 the propulsion of the blood through the arteries 

 and veins by artificial injection of fluids in a 

 dead animal, we observe that the jet from an 

 opened vessel continues to flow for some time 

 after we have ceased to drive the piston of the 

 syringe. The arteries being much stronger 

 than the veins, re-act with greater power than 

 they do against the distending force of the 

 heart. Were the arteries rigid tubes, it is ma- 

 nifest that in a given time just as much blood 

 would pass from their remote extremities into 

 the commencement of the veins, as enters them 

 by the mouth of the aorta; but the arteries 

 must be fuller at one time than another, for the 

 quantity of blood expelled from the ventricle at 

 each systole, must pass suddenly into the first 

 part of the aorta, while an equal quantity of 

 blood, which must necessarily pass from the 

 remote arteries into veins, as it moves uni- 

 formly, must employ the whole period of 

 time occupied by a complete action of the 

 heart in its passage; and consequently it is 

 manifest, ttat the arterial system must be fuller 

 just after than immediately before tlie contrac- 

 tion of the ventricle. The arteries are distensi- 

 ble and plastic, they yield a little to every suc- 



cessive stroke of the ventricle, and during the 

 diastole they re-act by their elasticity, so as to 

 keep up the flow of blood. We have already 

 said, in speaking of the heart, that the muscular 

 contraction of that organ is the chief, if not the 

 only source of the power propelling the blood. 

 It is only in those arteries which are nearest to 

 the heart, however, that the blood can be said to 

 be propelled by the direct impulse of the ven- 

 tricle, for in the rest of the arterial system, the 

 progression of the blood is immediately effected 

 by the elastic power of the arteries, called into 

 operation in consequence of their distension by 

 the action of the heart. In the experiments of 

 artificial injection of the bloodvessels in dead 

 animals already mentioned, as long as we con- 

 tinue to drive the piston of the syringe, and to 

 propel fluids through the arteries into the veins, 

 the arteries are kept in a state of forced disten- 

 sion ; in consequence of this, the fluid issues 

 from an opened artery with a jet accelerated 

 after each successive stroke of the piston, and 

 continues to flow for some time after the pro- 

 pelling power has ceased to act. The unifor- 

 mity of the stream of fluid from the veins, 

 which occurs in the same experiment, is a proof 

 that the continued flow of blood in these tubes 

 may, in the living body, be owing to an impul- 

 sion from the heart, transmitted by the arteries, 

 and that it is caused by the elasticity of the 

 coats of the vessels themselves. 



a. Velocity of' the blood in different arteries. 

 The space of the aorta filled up by the blood 

 propelled from the ventricle at each systole, 

 divided by the time occupied in its propul- 

 sion, constitutes the velocity of the blood in 

 the first part of the aorta. The diameter of the 

 aperture of the aorta at the ventricle being taken 

 as on an average 1-1 2 of an inch,* its area would 

 be one square inch, and consequently 1 oz. 

 which equal 2'45 cubic inches of blood, would 

 occupy a little more than 2*5 inches of the aorta, 

 supposing its size to be for such an extent of a 

 uniform diameter. As it is satisfactorily ascer- 

 tained by actual measurement, that the blood 

 contained in the smaller vessels is in much 

 greater quantity than that in the larger trunks ; 

 or, in other words, as the capacity of the smaller 

 vessels taken together is greater than that of 

 the larger, it will at once be apparent, that the 

 velocity of the blood must diminish in passing 

 from the larger to the smaller vessels. The 

 arterial and venous vessels may in fact be re- 

 garded as two hollow cones, curved so as to be 

 joined at their apices to the heart, and at their 

 bases to one another. The veins, being more 

 numerous and wider than the arteries, must be 

 represented by a wider cone. The section of 

 these cones at any place is supposed to give 

 the combined area of the section of the vessels 

 at a corresponding distance from the heart. 



The estimates made by different authors of 

 the relative velocity of the blood in the larger 

 and smaller vessels, differ in a great degree, 



* The aperture of the aorta is somewhat less 

 than one inch in diameter in most persons ; we 

 may, however, adopt the above estimate of its size, 

 as the sinus of the aorta is much wider than irs 

 aperture, 



