The velocity of the blood must not be confounded with the velocity of the pulse wave, which bears to it the same relation as the surface waves on a river do to the rate of the stream of water.

It has already been mentioned that the general bed of the blood increases from the aorta to the capillaries, and decreases from the capillaries to the vena cava. The branches or tributaries of an artery or vein have collectively a larger sectional area than the vessel from which they spring or to which they lead respectively; or, in other words, if we imagined the whole vascular system fused together into one tube it would form two somewhat irregular cones, one corresponding to the arteries and the other to the veins, with their bases placed at the capillaries and their apices at the heart. Between the two cones a still wider portion would represent the aggregate sectional area of the capillaries. (Fig. 128, p. 288).

Since the same quantity of blood must pass through each section of these cones in a given time, the rate at which it flows must vary greatly in the different parts, being faster in proportion as the diameter of the part is narrower, in accordance with the well-known physical law that with the same quantity of liquid flowing, its velocity changes inversely with the square of the diameter of the tube ( V 00 q/d2). Thus, the mean velocity of the flow in the arteries becomes slower as the capillaries are approached, and in the wide bed of the latter the rate of the current is reduced to a minimum. In the small veins the rate is slower than in the larger trunks, but on the venous side its rapidity never reaches that of the aorta, where it may be said to move at least twice as quickly as in the vena cava.

The following table may be useful in giving a general idea of the average velocity in different parts of the circulation: - 27

Near valves of aorta - while the ventricles are contracting it reaches

1200 mm. per sec.

Descending aorta,.......

. 300-600

Carotid,.......... .

. 205-357

Radial,............

. 100 mm. per sec.

Metatarsal

. 57

Arterioles,...........

. 50

Capillaries

. 0.5

Venous radicles

. 25

Small veins on dorsum of hand,. .

. 50

Vense Cavae

. 200

In the aorta near the valves the blood current varies in rapidity, because the flow through the aortic orifice is intermittent, and this variation must be more or less communicated to the neighboring arteries in the form of an increase of rapidity coincident with the beat of the arterial pulse. The variation in the rate of the blood flow which is caused by the heart beat diminishes with the force of the pulse as the smaller arteries are approached, and finally ceases completely in the capillaries, where under ordinary circumstances the flow is perfectly continuous. In the first part of the aorta the velocity of the blood flow is reduced to nil after each ventricular beat, while in the capillaries no change is perceived. Between these two extremes all gradations may be found, which follow the same rules as the pulse.

The general mean velocity varies directly with the blood pressure, which bears a generally inverse relation to the calibre of the arteries. The velocity in any one artery and its branches will vary with the calibre of those vessels, which are constantly undergoing local changes in size.

Generally speaking, quick heart beats cause increase in velocity of the stream, but no definite or invariable relation exists between the rate of the heart beat and the current of the blood. The vasomotor influences have, no doubt, much more effect than the heart beat on the rate of the stream in the smaller vessels the calibre of which they control.

In looking at the blood passing through the small vessels of a transparent tissue, such as the frog's tongue or web, it appears that different parts of the column of fluid move with different velocities. Down the centre of the stream the red corpuscles are seen coursing rapidly, while between the central part and the vessel wall on each side a pale line of plasma can be recognized, which seems to flow more slowly and to carry with it only a few white corpuscles.

Small portion of Frog's Web, very highly magnified.

Fig. 142. Small portion of Frog's Web, very highly magnified. {Huxley).

A. Wall of capillary vessels. B. Tissue lying between the capillaries. C. Epithelial cell of skin, only shown in part of specimen where the surface is in focus. D. Nuclei of epithelial cells. E. Pigment cells contracted. F. Red corpuscles (oval in the frog). G. H. Red corpuscles squeezing their way through a narrow capillary, showing their elasticity. I. White blood cells.

In the veins the velocity varies greatly with a variety of circumstances which have little or no effect on the arterial flow. Thus, the position of the body or limb, the activity of the neighboring muscles and the respiratory movements alter it, but as a general rule the flow in the veins is pretty steady, there being no pulsation or corresponding variation of velocity. In the large vessels the onward flow is affected by the contraction of the auricles. During the auricular systole the veins cannot empty themselves, and therefore there is a slight check to the onward flow, and the velocity of the current is correspondingly reduced. In cases where the auricles are dilated and distended with blood this may cause a definite pulsation, which becomes visible in the great veins of the neck.