CHAPTER II. (W.)

CIRCULATION AND RESPIRATION.

IN Man, as has been already stated in the Introduction, the circulation is double, and is carried on by a double heart, or, in fact, by two hearts, which, though united together in one mass, are yet entirely distinct in their office. One heart receives the blood by veins from the body, and sends it by arteries to the lungs; this is the right side of the organ. The other receives it by veins from the lungs, and sends it by arteries to the body; this is the left side. The blood thus makes two circuits, each beginning and terminating at the heart, and these two circuits make up the complete circulation. (Figs. 4 and 5.)

The time occupied for the circulation of any particular portion of blood, is astonishingly short. The heart expels about two ounces at every contraction; and of these about seventy-five take place in each minute. Supposing the quantity of the blood to be about thirty pounds, a quantity equal to this will pass through each side of the heart every three minutes, or twenty times an hour. But the motion of the blood is, under some circumstances, much more rapid than this; and there are facts which show that some portions of it may leave the heart and return to it in less than a minute.

The blood is not propelled through the arteries in a regular, even current, but by successive waves, which correspond to the contractions of the heart. Hence in a wound of one of these vessels, it flows in a series of jets, like the water from a forcingpump. This motion causes the pulse. From the veins, on the contrary, it flows in a steady, continued stream, as we see it in a person who is bled.

The mode in which the arteries are distributed over the body, is worthy of notice. All wounds of them are attended with danger, and a division of the larger is often speedily fatal from loss of blood. They are secured from this in great measure, by the position they are made to occupy, and the manner in which

they are distributed along the limbs. In the upper extremity, for instance, the main artery is protected by the collar-bone, under which it passes to the arm; thence it creeps down the inside of the bone, deeply buried beneath the flesh, to the elbow; and in a similar way, deeply covered by flesh, it goes down to the wrist. Everywhere the parts most exposed are free from large arteries, so that none but deep or unusual wounds are likely to reach them.

Another circumstance in this distribution is equally worthy of remark. It is obvious, if the vessel passed along the outside of a limb, or the outside of a joint, that, when the limb was bent, its coats would be put upon the stretch; it would be compressed, and the free passage of blood obstructed. This is prevented by the arrangement just described. For the same purpose, it does not run in a straight line, but has many turnings and windings in its course. This enables us to make violent and extensive motions without any compression, or strain of its coats, and in this way obstruction to the flow of blood, and injury to the vessels, are prevented.

The constant passage of the blood is necessary to the life and action of every organ. When interrupted, the part immediately ceases to perform its office, and if the interruption continue, it dies. So too, if the circulation of the vital organs be cut off, death of the whole body takes place at once; and hence it is, that ruptures or wounds or diseases of the heart cause death so suddenly.

But it is not only necessary that blood should be circulated to every part, but that this should be blood which has been subjected to the influence of the air in respiration. The venous or black blood is incapable of supporting life; and if respiration be suspended so that this is sent to the organs, instead of the red or arterial, the effect is as certainly fatal-though less speedily — as the suspension of the circulation altogether.

In the whole compass of Nature there is perhaps no single created thing which is so justly an object of attention and admiration as the heart, and this chiefly because its structure and mode of action are such that we almost perfectly understand them. To a cursory view it appears to be merely a mass of red flesh, not

larger than the hand, and yet by a careful examination of its structure, there is found within its walls a mechanism which keeps up the motion of the blood, and is necessary every moment to the life of the system. There is no organ in which we more clearly see the connection between the object intended and the means by which it is accomplished. There may be some differences of opinion about minor details in its mechanical action; but as to the general purpose, the play of its valves, the uses of its cavities, and the course of the blood through them, all is as clear as the movements of a common pump.

It is worthy of notice that the structure of those natural objects which most closely resemble instruments of human invention, produces upon us the most forcible impression as evidences of design in their creation. No organs, for example, are so frequently adduced by the Natural Theologian as illustrations to enforce the great truths he would establish, as the eye and the heart. There are none which, in the principles upon which they are constructed, resemble so nearly pieces of mechanism contrived by man. The eye is a perfect optical instrument, and may be very closely imitated, except as to that mysterious power by which it communicates with the mind. So too the heart, — although its power of contraction is derived from the vital forces, whose nature is equally mysterious, propels the blood upon purely hydrostatic principles; and its valves resemble, in the office they perform and the mode in which they determine the current of the blood, those which are constantly employed, for a great variety of purposes of an analogous nature, in the arts. It is only an object of greater admiration than the eye, inasmuch as its office is so much more important, and its connections with other parts concerned in the circulation so complicated and yet so clear.

It seems at first singular that those works of the Deity should excite so much of our admiration, which come the nearest to our own humble inventions. Yet it is an undoubted fact. It is explained by reflecting that in such cases we understand clearly the connection between the means used and the end obtained; and we understand it because the means are like those we have used, and the end analogous to those we have sought. In other organs it is not so. The conversion of food in the stomach and

intestines into blood, is as wonderful a process, so far as the end obtained is concerned, but we know very little of the manner in which the means employed bring it about. The heart almost declares its own office, but there is nothing in the structure of the stomach, or the known properties of the gastric juice, to suggest the remarkable transmutation they effect. No one object in Nature is probably in itself more indicative of creative power and wisdom than another. But we understand some far better than others; and in proportion as we comprehend, we admire.

By respiration the blood is exposed, in its passage through the lungs, to the influence of the air, and by this influence some change is effected, which restores the vitality it has lost in its circulation through the rest of the body.

By the air is meant that common elastic fluid which envelops the whole earth, and extends to a certain distance from its surface. It constitutes what is called the atmosphere. By its weight, its compressibility, and its pressure in all directions, it insinuates itself into every vacuity; and its presence is absolutely necessary to the existence of every vegetable and animal. In order, however, to understand the manner in which it contributes to the support of living things, it is necessary to know something of its composition.

Although the air, as we breathe it, seems to be a simple and homogeneous fluid, it is in fact composed of two distinct constituent or elementary parts, upon the mixture or combination of which its adaptation to the preservation of life depends. It contains, besides these, some other ingredients of minor importance. These main elements are two permanently elastic fluids or gases, called oxygen, and nitrogen or azote. Atmospheric air contains about twenty-three parts, by weight, of the former, and seventyseven of the latter, out of one hundred; or, since oxygen is the heavier of the two gases, twenty-one, by measure, of oxygen, and seventy-nine of azote It is upon the oxygen of the air, that its fitness for supporting animal life depends; for, when an animal is confined in a small quantity of air till this is exhausted, it dies from suffocation, although the azote remains unaltered.

No animal can exist in an active state without air, but different classes of animals differ very much as to the manner in which

the function of respiration is performed. The influence which the air exerts upon the blood, produces some change or imparts some principle, which renders it fit to be distributed to the body for its nourishment. In all the animals with red blood, namely, Mammalia, Birds, Reptiles, and Fishes, this change consists, so far as can be observed, in imparting to the dark-red or venous blood, which is sent to the lungs by the heart, a bright red or vermilion color. In this state, it is returned to the heart, and thence distributed throughout the body by the arteries.

In the Mammalia, the air is alternately drawn into the lungs, and expelled from them, by the action of the diaphragm and muscles of the ribs and abdomen. This is called the inspiration and expiration of the air, and is constantly going on. In the greater part of the animals of this class, if this process be stopped but for a few moments, death is the inevitable consequence; but in some species it may be suspended for a longer period. This is the case with the seal and the whale. Even men may acquire by habit the power of existing a considerable time without breathing, as is the case with the fishermen who dive for pearls; but many of the stories which have been related with regard to this subject are probably destitute of foundation.*

There are many other kinds of air or gas, which may be taken into the lungs, beside the atmospheric; but no other will support life. Even pure oxygen, and nitrous oxide, another gas containing oxygen, although they may be breathed longer than any other kinds of air, will finally prove fatal. It is only when oxygen is combined with azote in the proportions above mentioned, that it is adequate to the continued support of life.†

* Very marvellous accounts have been formerly given of the length of time during which persons have remained under water without death. The most reliable accounts of the performances of the pearl-fishers render it probable that they cannot endure an entire suspension of respiration of more than a minute and a half or two minutes.

† Water destroys the life of animals, merely by preventing the admission of air; it does not itself enter the lungs, or at most only in a very small quantity. There are some gases which operate in the same way. The windpipe is spasmodically closed against them, and they do not enter the lungs; such are carbonic acid gas, ammoniacal gas, chlorine or oxymuriatic gas, &c., when unmixed. Some other gases are inspired with sufficient ease, but produce death, either merely for the want of oxygen, as hydrogen and pure azote; or, in a cer