one limb and rise in the other. mercury stands in the two limbs gives the pressure of mercury equal to the pressure of blood in the case examined. If the column of mercury be closely watched, or if a float carrying a pen be arranged so as to record the level of the column upon a revolving cylinder, it will be noticed that the height of the column is variable-irregularly variable, i.e. rising or falling more or less without obvious cause or in consequence of muscular efforts; and regularly variable, rising and falling with each beat of the heart and with each act of respiration. The complete apparatus for taking blood-pressure records is commonly termed a kymograph.

The difference of level at which the

Arterial blood-pressure cannot be studied upon man by the method above described; all direct evidence on the subject is derived from experiments on animals, such as the dog, rabbit, and horse, and from the facts thus obtained we judge of what must take place in the human arterial system. The mean blood-pressure in the carotid artery of a rabbit is equal to about 8 to 10 cm. of mercury, of a dog or horse 12 to 15 to 20 cm.; in the human carotid artery the pressure is supposed to have about the same value, viz. about 15 to 20 cm. (=6 to 8 inches).

Attempts are, indeed, made to estimate arterial pressure clinically by means of the sphygmograph and other instruments, such as the plethysmograph and the sphygmomanometer, but it is difficult to get exact results by any instrument, and the pulse skilfully felt is the readiest means of estimating arterial pressure on man. By feeling the pulse, we may ascertain that bloodpressure is unusually high, or unusually low, or not far wrong; we say, accordingly, that tension is high or low or normal, and we speak of a pulse as of high tension, or of low tension, or of normal tension.'

We have recognised as obvious that, cæteris paribus, arterial pressure should be greater with greater heart's force, less with less heart's force, greater with greater peripheral resistance, less with less peripheral resistance. All these statements may be illustrated by experiments and observations. Violent exertion causes the heart to beat more frequently (and possibly each beat

The word 'tension' is more commonly used in clinical medicine than the wordpressure.' Arterial tension signifies the elastic force exerted by a distended artery; this elastic force is equal to the distending force of blood-pressure. The terms' arterial tension,'' arterial pressure' may therefore be substituted each for the other. The term 'blood-pressure,' used without qualification, is understood to denote arterial blood-pressure.

FIG. 21.-DIAGRAM SKETCH OF THE MERCURIAL KYMOGRAPH. (Hering's Model.)

FIG. 22.-PORTION OF A BLOOD-PRESSURE TRACING FROM THE CAROTID ARTERY OF A RABBIT. The small undulations are due to the heart-beat; the larger undulations upor which they are superposed are due to the movements of respiration. The height above the zero line is one-half the blood-pressure expressed in terms of mercurial pressure.

may be stronger, though this is not necessarily so), but in any case the total force exerted by the heart during a given timesay a minute-is greater than usual, and the blood-pressure is temporarily increased; a sudden shock, or a strong emotion, or experimental stimulation of the vagus nerve, weakens or arrests the action of the heart, and the blood-pressure is temporarily diminished. If the vasomotor centre is stimulated, the muscular arterioles contract and narrow the outlet from the arterial system, so that the peripheral resistance is increased; blood-pressure is raised. If the vasomotor centre is destroyed, or its action is depressed by shock, the muscular arterioles relax and widen the arterial outlet; blood-pressure falls.

Blood-flow.—All these are extreme cases, which actually do occur in the living body; but we have to recognise that the several alterations, whether of the heart or of the arteries, naturally limit and neutralise each other, so that a marked change of pressure, as above described, can only occur in consequence of an excessive alteration of one of two factors. The co-operative variations of the two factors will be better understood by taking into account how the rapidity of the blood-flow alters with the alterations of the heart or of the vessels. For the sake of brevity, these alterations may be cast into tabular form by the side of the alterations of blood-pressure which we are discussing. The table includes all the possible variations which may theoretically take place, the consequences in each case being denoted by the signs or, to signify increase or + diminution.

The first four cases need not detain us long. If-supposing the heart's force to remain constant-the arterioles contract and increase the peripheral resistance, it is obvious that pressure will be raised, and that through the constricted outlet less blood will flow (1); vice versâ, if the arterioles dilate, widening the

outlet and diminishing the resistance, it is clear that pressure will fall, and more blood will flow through the vessels (2). Ifsupposing the peripheral arterioles to remain unaltered-the heart beats more strongly or less strongly, it is obvious that with more force there will be increased pressure and increased flow of blood (3), with less force there will be diminished pressure and diminished flow (4).

Turning to the next four cases, in which both factors are altered, it is evident that in (5) increased heart's force will give more pressure, while diminished peripheral resistance will give less pressure; these alterations neutralise each other, and pressure remains unaltered, or, if either should exceed the other, the resultant alteration, being only their difference, is not great. But as regards blood-flow the result is otherwise; increased heart's force gives increased flow, diminished peripheral resistance also gives increased flow, and these two alterations united give a resultant alteration which is comparatively great. Similar reasoning applies to the combination (6), all data and results being reversed; thus, diminished force of the heart gives less pressure and less flow, increased resistance gives greater pressure and less flow; as our resultants we have little or no alteration of pressure, and a great diminution of the blood-flow. Thus, in these two cases (5 and 6) we have a minimum alteration of blood-pressure with a maximum alteration of blood-flow. If now we consider the last two cases, we shall find how dif ferently the results come out. In combination (7) the increased heart's force gives increased blood-pressure, and so does the increased peripheral resistance; the resultant increase of bloodpressure is therefore considerable, while as regards the bloodflow the first factor increases, the second diminishes it, and the resultant alteration is little or nothing. In combination (8) similar reasoning applies; both factors diminish the bloodpressure, but the first diminishes while the second increases the blood-flow. Thus in these two cases we have a maximum alteration of blood-pressure with a minimum alteration of bloodflow.

The body, as a whole, requires, according to circumstances, more or less blood; more or less blood-pressure is of itself useless, and accordingly we find that the temporary variations which naturally occur in the body are such as produce a minimum alteration of blood-pressure with a maximum alteration of blood-flow (viz. cases 5 and 6), whereas the converse variations

(viz. 7 and 8), giving a maximum alteration of blood-pressure with a minimum alteration of blood-flow, are abnormal and do not occur in the healthy body. In diseased conditions they do, however, occur, and persist as enduring states: in renal diseases a strongly beating heart with constricted arterioles and consequently high blood-pressure, form a very usual combination; in great prostration from any cause, a weak heart with relaxed arterioles, and consequently low blood-pressure, is the invariable rule. And it may be incidentally noticed how in these cases the two factors add themselves as regards blood-pressure but oppose and compensate each other as regards blood-flow, so that the body is not starved of blood in the same proportion as the circulation becomes weaker (case 8); nor over-supplied with blood in proportion to a heightened blood-pressure (case 7). Referring to the table, we see that in these combinations a minimum alteration of blood-flow accompanies a maximum alteration of blood-pressure. Thus analysed we can recognise the economy of the relationship between the state of the heart and of the vessels, and between blood-pressure and blood-supply. The essential event is blood-supply, to which blood-pressure is subservient. Normal fluctuations of tissue-activity elicit corresponding fluctuations of blood-supply with minimum alterations of pressure. Abnormal alterations of blood-pressure do not, until they are excessive, interfere with a normal and necessary rate of blood-supply.

It should, however, be added that the immediate effects of increased or diminished blood-pressure are always an increase or a diminution of blood-flow, although in the first case the vessels are constricted and in the second case dilated. This result, which could not have been foreseen on a priori grounds, has been established by direct measurement of the aortic bloodcurrent. Destruction of the spinal bulb relaxes the arterioles, lowers blood-pressure, and reduces blood-flow; excitation of the bulb constricts the arterioles, raises blood-pressure, and accelerates blood-flow.

Local variations.-The general requirements of the whole body vary, and are met, as above described, by variations of the general blood-pressure and blood-flow, variations of flow being more important than variations of pressure. We have further to consider local variations in response to the particular requirements of its several parts or organs. Two different parts or organs may at the same time require very different amounts of