MOVEMENTS OF THE HEART.

139

of their veins, the effect passing quickly forward; the ventricles contract more slowly, but simultaneously in every part.

During each beat of the heart two sounds may be heard, followed by a silence. The first sound is dull; the second, which fol- Sounds of the lows it quickly, is sharp. They may be imitated by artic- heart. ulating the syllables lubb, dup. The first is due to the contraction of the muscular fibres of the ventricles, and the striking of the apex of the heart against the wall of the chest; to a certain extent, the opening of the semilunar valves, and the rush of the blood into the pulmonary artery and aorta contribute to it. The second sound is due to the shutting of the semilunar valves of the aorta and pulmonary artery.

At each contraction of the ventricles the heart strikes against the walls of the chest, usually between the fifth and sixth ribs, and an inch or two to the left of the sternum. This motion is partly due to the action of the spiral muscular fibres of the ventricles, which gives a tilt to the heart, and partly to the globular form which the whole organ suddenly

assumes.

The number of pulsations made by the heart differs very much at different periods of life: at birth it is from 130 to 140 per Number of pulminute; at the seventh year, from 80 to 85; during mature sations. life, from 70 to 75; and in old age, from 50 to 65. In females it is more frequent than in males. It observes a general relation with the number of respirations, five pulsations commonly occurring during one respiration. It varies with incidental circumstances. During sleep it declines in frequency; after eating, or during exercise, it is quickened. Examined from morning to evening, it becomes slower by degrees. Lying down, the pulse is slower; in a sitting posture, more frequent; and still more so when standing, the variations depending on muscular exertion. In conditions of disease, the ratio between the number of pulsations and respirations is variable.

Structure and

walls.

The walls of the left ventricle are twice as thick as those of the right, and the force of its contractions is about double. The capacity of the two ventricles is nearly the same, and is taken power of the at about three ounces. The active force with which the auricles dilate is feeble, and wholly incompetent to exert any thing like the suction power at one time supposed, yet that they are not distended by the mere influx of the blood is satisfactorily proved by their dilatation after the heart has been cut out.

With respect to the absolute force which the left ventricle exerts for the propulsion of the blood into the systemic arteries, it is stated to be 13 lbs. This result is derived from the consideration that the pressure of the blood in the aorta is about 4 lbs. 3 oz.

That the motions of the heart can not be referred to the presence of the

140

Cause of the

heart.

CAUSE OF THE MOTIONS OF THE HEART.

blood, or any reflex action arising from the cerebro-spinal motions of the system, but must be attributed to the organ itself, is proved by their continuance after its excision from the body, or even after it has been cut in pieces. Some have supposed that the minute sympathetic ganglia with which it is furnished are the source of the motive power; others are disposed to impute it to a self-contractile power of its muscular fibres, irrespective of any nervous agency. Of course, it is admitted by all that the brain and spinal cord can influence these movements, but such effects are superadded and not uniform.

Of these opinions, we shall find many reasons for preferring the first when we come to the description of the nervous mechanism. It will be then seen that one of the prominent functions of nervous ganglia of a certain order, and particularly the ganglia of the sympathetic, is the storing up of impressions they have received, and thus becoming reservoirs or magazines of force. The power thus engendered or contained in them is by no means always delivered out in totality at once, but it may be in small portions, at intervals, for a long time; and doubtless in this way the minute sympathetic ganglia of the substance of the heart retain a power of keeping up the motions of that organ for a certain period of time, even though great lesions or morbid changes may have supervened. Such a mechanism recalls the manner in which chronometers are kept going during the short time that the action of the main-spring is taken off when the watch is wound up.

the arteries.

2d. The arteries are tubes consisting of different tunics or layers vaDescription of riously numbered by anatomists, but which may be sufficiently described as, 1st. The exterior tunic, containing fibres generally running lengthwise, connective and elastic tissue: it is of about the same thickness as the tunic below; 2d. The middle tunic, characterized by being composed of non-striated muscular fibres circularly arranged; 3d. The interior tunic, which is thin, and consists of a cellular or epithelial layer, smooth and polished, to permit of the ready passage

of the blood.

The elasticity of the arteries enables them to sustain the sudden action of the heart by distending to a certain degree as the blood is driven into them, and by their gradual collapse when the ventricles cease their pressure, the jetting or intermitting flow is converted eventually into a continuous stream. The mechanical influence of the heart is thus decomposed into two portions: one, which is of momentary duration, or, at all events, lasting only so long as the ventricle contracts; and a second, which is occupied in distending the elastic arterial tube; but this portion is not lost to the circulation, since the tube, as it contracts, yields it back again to the blood. The momentary impulse of the heart is thus spread over a considerable duration without loss.

ACTION OF THE ARTERIES.

141

The muscularity of the arteries is shown by their contraction on exposure, their subsequent dilatation being due to their elasticity, this contractile property being continued for some time after death. It is also proved by the great diminution of diameter which arteries exhibit when under the influence of an electric current. The quantity of muscular and elastic tissue in different arterial tubes is usually in an inverse proportion. In the great arteries the elastic tissue abounds, in the smaller the muscular increases. By their muscular coat the quantity of blood in these tubes can, within certain limits, be regulated.

At each injection of blood into it an artery distends. It then contracts, and thus gives origin to a pulsation. Its increase is Action of the both in diameter and length, the tendency being to lift it at arteries. cach pulsation. The distention does not occur at the same instant in all these tubes, but those nearest to the heart yield first, and the more distant a little later. There is therefore what may be termed a wave of distention passing throughout the length of each arterial tube, and another actual wave in the blood itself. These pass onward at different rates of speed. The interval of wave-motion from the heart to the wrist is about one seventh of a second. Of course this wave-motion is to be distinguished from the absolute movement of the blood, which is much slower. In the carotid artery the flow of the blood is about one foot in one second.

A pressure or impact, communicated to a liquid in a long tube, is transmitted to the more distant end with vastly more rapidity than the liquid itself could flow through the same distance. Thus, if we were to suppose a very long metal tube to be filled completely with water, its two ends having been tightly closed by tying pieces of bladder over them, the tap of a finger on one of the pieces of bladder would be almost instantly felt by a finger laid on the other. Indeed, it has been proposed to establish telegraphic communication on this principle, though such attempts would prove abortive from the interference of collateral circumstances. This example may serve, however, to illustrate the essential difference between the flow of a liquid in a tube and the passage of a pulsation through such a liquid contained in such a tube.

The capillaries may be regarded as tubular continuations of the arteries and the commencement of the veins. They ramify The capillaries. through the organic structures. They are of pretty uniform diameter, and may therefore be looked upon as cylinders. Their usual size is about of an inch; their mode of distribution varies with the structure and functions of the part they occur in: thus, in muscles they run parallel; in the papillæ they are looped.

They consist essentially of a delicate structureless membrane, analogous to cell membrane, and the sarcolemma of voluntary muscles. It

possesses a certain degree of elasticity, and presents here and there cell nuclei.

The interspaces between adjacent capillaries vary much in size and Size of inter- shape, the latter variation being dependent on the mode of spaces. distribution, whether parallel, reticulated, looped, &c.; as to size, in the liver the interspaces are of less diameter than the capillaries, in the choroid coat still smaller, but in the cellular coat of the arteries they are ten times larger than the vessels. These interstitial spaces arc nourished by the matter which exudes through the thin walls of the capillaries.

STRUCTURE OF THE VEINS.

143

taken at three inches per minute, that through the pulmo- Motion of the nary being five times as quick, the length of the capillary tube blood in the capillaries. to be passed of an inch, so that the passage from the artery to the vein may be accomplished in less than one second. It is to be remarked, however, that all parts of the cylindrical stream do not move with equal rapidity. Those parts which are nearest to the wall of the vessel are spoken of as the still layer, from their tardy movement. It is in this that the white corpuscles may be seen.

a

Fig. 64.

a

White corpuscles in the still layer.

b

b

Fig. 64 shows a portion of a small vessel from a frog's foot: a, a, red blood elliptic cells, occupying the axis of the vessel, and exterior to them, moving more slowly, or occupying the still layer, the white spherical cells; b, b, nucleated epithelium.

4th. The veins have a structure in some respects different from The veins: that of the arteries. Their their structure. elastic coat is by no means so much developed, and their muscularity less distinct. With the exception of those of the lungs, abdominal viscera, and brain, their interior is furnished with valves of single, double, or triple flaps, in all instances opening toward the heart. The blood flows equably in them, the pulsating action of the ventricles having disappeared in the capillaries. Since they present an aggregate capacity two or three times that of the arteries, the motion of the circulation in them is proportionally slower. Fig. 65 is a diagram showing the manner in which the valves open when the blood flows in the course indicated by the arrows.

Fig. 65.

Valves of veins open.

Fig. 66.

Valves of veins shut.

Fig. 66 shows their application to each other, or to the sides of the vein, and the consequent bulging of that vessel when the current, as indicated by the arrows, is in the opposite direction.

Having now described the structure and action of the heart, the arteries, capillaries, and veins respectively, as far as is necessary, it remains to group those actions together, and present the theory of the circulation at one view.

trine that the

heart is the sole

But, before entering on this, it is proper to offer an ar- Error of the docgument against the doctrine of those physiologists who still maintain that the circulation is wholly dependent on the heart, and that that organ is entirely competent to carry it on.

cause of the cir

culation.