136 THE VASCULAR MECHANISM. 



the arteries, and possess a very variable amount of muscular tissue ; they 

 collapse when the veins are empty. Though all veins are more or less elastic 

 and some veins are distinctly muscular, the veins as a whole cannot, like the 

 arteries, be characterized as eminently elastic and contractile tubes ; they 

 are rather to be regarded as simple channels for conveying the blood from 

 the capillaries to the heart, having just so much elasticity as will enable 

 them to accommodate themselves to the quantity of blood passing through 

 them, the same vein being at one time full and distended, and at another 

 time empty and shrunk, and only gifted with any great amount of muscular 

 contractility in special cases for special reasons. The united sectional area 

 of the veins, like that of the arteries, diminishes from the capillaries to the 

 heart ; but the united sectional area of the venss cavse at their junction with 

 the right auricle is greater than, nearly twice as great as, that of the aorta 

 at its origin. The total capacity also of the veins is much greater than that 

 of the arteries. The veins alone can hold the total mass of blood which in 

 life is distributed over both arteries and veins. Indeed, nearly the whole 

 blood is capable of being received by what is merely a part of the venous 

 system, viz., the vena porta and its branches. 



THE MAIN FACTS OF THE CIRCULATION. 



102. Before we attempt to study in detail the working of these several 

 parts of the mechanism, it will be well, even at the risk of some future repe- 

 tition, to take a very brief survey of some of the salient points. 



At each beat of the heart, which in man is repeated about 72 times a 

 minute, the contraction or systole of the ventricles drives a certain quantity 

 of blood, probably amounting to about 180 c.c. (4 to 6 oz.), with very great 

 force into the aorta (and the same quantity of blood with less force into the 

 pulmonary artery). The discharge of blood from the ventricle into the 

 aorta is very rapid, and the time taken up by it is, as we shall see, much 

 less than the time which intervenes between it and the next discharge 

 of the next beat. So that the flow from the heart into the arteries is 

 most distinctly intermittent, sudden rapid discharges alternating with 

 relatively long intervals during which the arteries receive no blood from 

 the heart. 



At each beat of the heart just as much blood flows, as we shall see, from 

 the veins into the right auricle as escapes from the left ventricle into the 

 aorta ; but, as we shall also see, this inflow is much slower, takes a longer 

 time, than the discharge from the ventricle. 



When the finger is placed on an artery in the living body a sense of resist- 

 ance is felt, and this resistance seems to be increased at intervals, correspond- 

 ing to the heart-beats, the artery at each heart-beat being felt to rise up or 

 expand under the finger, constituting what we shall study hereafter as the 

 pulse. In certain arteries this pulse may be seen by the eye. When the 

 finger is similarly placed on a corresponding vein very little resistance is felt, 

 and, under ordinary circumstances, no pulse can be perceived by the touch 

 or by the eye. 



When an artery is severed, the flow of blood from the proximal cut end, 

 that on the heart side, is not equable, but comes in jets, corresponding to the 

 heart-beats, though the flow does not cease between the jets. The blood is 

 ejected with considerable force, and may in a large artery of a large animal 

 be spurted out to the distance of some feet. The larger the artery and the 

 nearer to the heart, the greater the force with which the blood issues, and 

 the more marked the intermittence of the flow. The flow from the distal cut 

 end, that away from the heart, may be very slight or may take place with 



