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tion of its veins; it may be diminished by constriction of its minute arteries, or by obstruction of its main artery. Generally speaking, the volume of a part or organ increases and diminishes with its functional

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Simultaneous tracing of spleen volume (upper line) and of arterial pressure (lower line); rhythmical contractions of the spleen without sensible variations of arterial pressure. (Roy.)

activity, greater activity being accompanied by dilatation of the vessels, less activity or rest by constriction of the vessels. These are active variations effected by the vasomotor nerves of the part itself. But

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passive variations may also be produced, in consequence of variations of the general blood-pressure brought about by vasomotor nerves in other parts of the body. The kidney, for instance, may shrink or swell with a rise or with a fall of blood-pressure. If it shrinks with a rise

FIG. 38.

Simultaneous tracing of kidney volume (upper line) and of arterial pressure (lower line); the large undulations are Traube-Hering effects; with the rise and fall of arterial pressure the kidney volume falls and rises. (Roy.)

or swells with a fall, these variations are local active changes due to constriction or to dilatation. If it shrinks with a fall and swells with a rise (as occurs after section of the renal nerves), these variations are

local passive changes of an organ more or less distended by the general blood-pressure. And when the nerves are intact the volume of the organ may either vary in the same sense as the general pressure, or in an opposite sense, according as the general pressure-changes overbear the local actions, or as the local actions exceed the general vasomotor changes of which they may form part. Other organs which are not directly influenced to any appreciable extent by vasomotor nerves, as, for instance, the brain, do not actively swell and shrink; the passive variations-distension by high blood-pressure, collapse of volume with low blood-pressure-are, on the contrary, well-marked and uncomplicated effects. The spleen, on the other hand, varies in most complicated fashion; it is a very distensible organ, and can therefore undergo considerable passive changes; it is abundantly supplied with vasomotor nerves, and has therefore a considerable range of active vascular variations of volume; and thirdly, it is highly contractile by virtue of its framework of involuntary muscle, so that, independently of passive and vasomotor variations, it can rhythmically dilate and contract.

The increase of volume produced at each heart-beat is an indication of the accelerated blood-flow then occurring; the venous outflow from the limb being constant, it is evident that a pulsatile increase of volume must be a measure of a pulsatile acceleration of arterial inflow. (Fick, v. Kries).

The most notable of Mosso's plethysmograph experiments is that intended to demonstrate a variation of cerebral circulation coincident with cerebral exertion; Mosso found that the volume of the arm was diminished during the performance of a calculation or other mental effort, and concluded that the effect was due to increased blood-supply to the brain; the results of the experiment are, however, not sufficiently regular to bear out this conclusion; alterations of volume of the arm, when they occur at all, are probably due to alterations of respiratory movements.

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FIG. 39.-CARDIOMETER TRACING. (Roy and Adami.)

The cardiometer.-A volume-recorder on the same principle has also been adapted by Roy to the contracting heart, and by its means the output of blood per contraction or per minute may be measured,

as well as the more or less distended state of the organ in various circumstances. Roy and Adami found, e.g., that during vagus slowing the ventricles became distended, the 'output' per contraction increased, but the output per minute diminished. Another point of importance definitely ascertained by means of the cardiometer is that the ventricles do not become completely emptied at each contraction; with high arterial pressure, as in asphyxia, the residual blood in the ventricles is at its greatest.

The sphygmomanometer is an instrument by means of which the pulse-tension can be approximately measured on man; the principle upon which it is based being the determination of the amount of -counter-pressure which is just sufficient to extinguish the pulse. A simple form of the instrument is as follows: an elastic finger-stall distended with air and connected with a mercury manometer is squeezed down upon the radial, or preferably the temporal, artery until the pulse is felt to vanish beyond the point of compression (recurrent pulsation in the radial being, if necessary, eliminated by compression of the ulnar artery); the pressure of air within the finger-stall as indicated by the manometer now just exceeds the blood-pressure within the artery during cardiac systole.

The plethysmograph has also been employed for the same purpose, the principle of counter-pressure being utilised as follows: pressure of the fluid surrounding the limb (arm, hand, or finger) is raised until a point is reached at which the pulsatile alterations of volume are at a maximum; this point is taken to indicate when the pressure of fluid on the limb is above the intra-arterial blood-pressure during cardiac diastole, but below it during cardiac systole.

The pulse. At each beat of the heart about four ounces of blood (or 120 c.c.) are forced by the left ventricle into the aorta and added to the mass of blood which is being pressed onwards throughout the arterial system. This sudden addition to the contents of an arterial system which is already distended, gives rise to a pressure-wave throughout all the arteries of the body, called the arterial pulse, which can be felt in any superficial artery. The most readily accessible artery for this purpose is the radial; other accessible arteries are the temporal, carotid, axillary, brachial, femoral, popliteal, anterior and posterior tibials, in all of which the arterial pulse may be felt, and, if desired, examined by means of recording instruments. The practical value of the pulse is that it affords means of judging of the state of the circulation; counting the pulse is the readiest means of ascertaining the frequency of the heart's beats, and the careful study of the tension of the pulse affords valuable information regarding the state of the arteries; it enables us to judge whether the

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general arterial pressure is high or low or about normal. The normal frequency of the pulse (i.e. of the heart's beat) is about seventy per minute in the male, eighty per minute in the female, and still higher in children; in the fœtus before birth it is 130 to 140. The frequency is increased by exertion, by taking food, by alcoholic stimulants, by some forms of emotion, in fevers, and in all kinds of debilitating disease. It is diminished during rest, during sleep, in some forms of emotion, in cerebral coma. As regards emotion, it is generally pleasurable and exciting emotion that raises the pulse-frequency, painful and depressing emotion that lowers it. As regards exertion, even the slight differences of exertion which insensibly accompany differences of posture affect the pulse-frequency; thus, for instance, in the same person the pulse has been counted in the lying posture 70, sitting 75, standing 80, and after a short run 120; in a weak or sick person the differences with posture may considerably exceed those observed in health; thus the difference in the sitting and lying postures may be twenty beats; it is therefore worth noting, when it is possible, what that difference is, for the number gives some measure of the weakness of the subject. Temperature also affects the pulse-frequency-high temperature raises it, low temperature lowers it; in the same person with a normal pulsefrequency of 72, the pulse has been counted in the hot chamber of the Turkish bath 96 per minute, and in a cold bath 60 per minute.

The same

The pulse may feel large or it may feel small; these terms signify that the impression is conveyed to the observer of a large or of a small wave of blood passing under his fingers as they rest upon the artery; but the terms are apt to mislead, as will presently appear from other considerations. remark applies to the terms strong and weak, for an apparently 'strong' pulse is common when arterial pressure is low, an apparently weak' pulse when arterial pressure is high. It is more important to pay regard to the compressibility of the pulse and to speak of the hard pulse or the soft pulse. A hard pulse is one which requires considerable pressure of the fingers upon the artery to obliterate; the artery feels distended between the beats, and the more the fingers compress it-within certain limits the more forcible does the beat appear. A soft pulse is easily obliterated by compression, and appears most forcible when the fingers are lightly applied. The hard pulse is a sign of high arterial tension or pressure, the soft pulse is a sign of low arterial

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