Chemical and Vital Processes.

855

sidered that there is a power of selection and transposition of these mineral substances which is incessantly going on in the living animal. We witness in other animals the same wonderful series of transformations. The blood of the cow is converted into a white, oily, saccharine liquid, milk, wholly different from blood in its physical and chemical properties. In the young animal, the calf, this white liquid serves as nutriment, producing blood, flesh, bone, fat, and all the other constituents of the body.

1091. It is the more necessary to insist upon these characters of living bodies, because some modern physicists and chemists of repute profess to see in life nothing but physical and chemical forces. An eminent philosopher has compared a living animal to a voltaic battery. So long as the battery is supplied with zinc and acids, a current of force is set up by which marvellous results are obtained. Withdraw these materials and all action ceases. In the living animal the food represents, according to this hypothesis, the zinc and acid, and the brain and nervous system the medium by and through which the so-called "vital" energy is manifested. Remove the food and you have what is called death.

A chemist of repute has treated the idea of the existence of any vital force, independently of chemistry and physics, as an obsolete dogma no longer received in modern science. He rests this view upon the fact that some substances, such as certain acids and principles hitherto called organic, have been artificially produced by chemical processes and quite irrespective of any so-called vital force; and from a few instances of this kind, he draws the hasty conclusion that in time all the constituents of the living body will be obtained by chemical processes in the laboratory.

It has always appeared to the writer that there is a great fallacy in this mode of reasoning. With regard to the illustration from the Voltaic battery, the points in which the comparison utterly fails have been studiously ignored. The living machine can not only go on working and at the same time repairing itself when needed, but it can multiply itself and produce an endless succession of similar machines invested with similar powers to its own. So, again, assuming that a chemist can produce artificially a liquid like albumen, a solid like starch, or a red colouring matter resembling blood, he is still very far from having solved the problem by which living is separated from dead matter. The vital force in the vegetable and animal not only produces these substances, with certain chemical properties, but invariably arranges them in a certain form.

856

Motion of the Blood in the Arteries.

They are organized as well as organic. The albumen presents itself in the living body in the form of feathers, hair, nail, and tortoise-shell. No physical or chemical energy, however applied, has yet succeeded in converting albumen into a feather, or starch into a granule of complex shape and structure, or in creating a single blood-cell out of any kind of red colouring matter.

It has been well observed that the limits of stature and growth in animals and vegetables, the persistence of form, and the reparative power manifested by the spontaneous restoration of injured or damaged parts, are forces or energies wholly different from those physical and chemical forces which govern inorganic matter. Why does the animal or vegetable grow, and why, if it once grows, does it ever cease to grow? Inorganic bodies are subject to physical and chemical changes, but we see in them nothing analogous to growth, maturity, and decay-stages which are inseparably connected with and invariably follow each other in bodies endowed with life. Other questions naturally arise out of these, which show in the strongest possible light, the existence of forces or energies, whether called "vital" or by any other name, wholly different from those of physics and chemistry. Why, in the process of growth, do bones remain bones, arteries, arteries, and nerves, nerves; and why is growth invariably controlled in its degree by the use or purpose to which the parts are put? These questions admit of no answer, so far as the laws of physics and chemistry are concerned. They are referable only to another force or energy wholly different in kind.

Force of the heart and motion of the blood in the Arteries.

1092. The contractions of the heart inject the blood into the arteries with a force maintaining such a tension in them, that, according to the interesting experiments of Dr. Hales, recorded in his Statical Essays, if any artery of a large animal like a horse be made to communicate with an upright tube, the blood will ascend in the tube to the height of about ten feet above the level of the heart, and will there continue, rising and falling a few inches with each pulsation of the heart. Now a column of ten feet, as explained in Art. 418, indicates a pressure of about four and a half pounds on a square inch of surface: this, therefore, is the force of the heart urging the blood along the arteries and through the capillaries into the veins. Recent experiments by physiologists have led to nearly similar results. The static force with which the blood is impelled into the

The Capillary Circulation.

857

human aorta, i calculated to be equal to four pounds four ounces, and that in the radial artery at the human wrist at only four drachms. The tension of the veins is much less, because of the resistance offered by the capillaries, and because the blood readily escapes from the veins into the heart. It is in the capillaries that the chief resistance is offered to the progress of the blood, for in them. there is greatly increased friction by reason of the increase of surface with which it is brought in contact, and as the stream is widened its velocity is diminished. Hales found that in a tube communicating with a vein, the blood stood only a few inches higher than the level of the heart. In small animals he ascertained the tension of artery and vein to be less than in large ones; and the ratios deduced for the human body, under ordinary circumstances, were eight feet column, or nearly four pounds per inch, for the arteries; and half a foot column, or a quarter of a pound per inch, for the veins. The least pressure on the top of either column will lift up the other; so, when the body is erect, the least pressure on the column of arterial blood may lift up the venous blood, and, were it not for the valves, the least pressure on the venous might lift up the arterial column. .

Passage of the blood through the Capillaries.

1093. We have seen above that the heart keeps up a tension or pressure in the arteries of about four pounds on the square inch of their surface; and with this force, therefore, is propelling the blood into the capillaries. If these last were passive tubes, constantly open, such force would be sufficient to press the blood through them with a certain uniform velocity: but they are vessels of great and varying activity: it is among them that the nutrition and repair of the different textures of the body take place, and that all the secretions from the blood are performed, as of bile, gastric juice or saliva; and to perform such varied and often fluctuating offices, they require to be able to control, in all ways, the motion of the blood passing through them. The capillaries of the cheek, under the influence of shame, dilate instantly, or lose their ordinary contractile power, and admit more blood, producing what is called a blush-while under the influence of anger or fear, they suddenly contract and empty themselves, and the countenance becomes pallid -tears or saliva, under certain circumstances, gush in a moment, and in a moment again are arrested. The action of cylindrical vessels, capable of causing these phenomena, depends on a con.

858

Circulation in the Veins.

traction and dilatation of their coats under a special system of nerves, called vaso-motor.

A muscular capillary tube, strong enough to shut itself against the arterial current from the heart, is strong enough also to propel the blood to the heart again through the veins, even if the resistance on the side of the veins were as great as the force on the side of the arteries. For if we suppose the first circular fibre of the minute tube` to close itself completely, it would, of course, be exerting the same repellant force on both sides, or as regarded both the artery and vein. If, then, the series of such fibres forming the tube were to contract in succession towards the vein, as the fibres of the intestinal canal contract in propelling the contents of that canal, it is evident that all the blood in the capillary would thereby be pressed into the vein towards the heart. If after this the capillary again relaxed on the side of the artery, so as to admit more blood, and again contracted towards the vein as before, it would produce a forward motion of the blood, first towards the vein, and then in it, independently of the heart, and might carry on a slow circulation if there were no heart.

Passage of the blood through the Veins.

1094. The veins have much thinner coats than the arteries, and, if taken altogether, have much greater capacity: for besides being larger than the corresponding arteries, they exist, in many situations, as double sets, an exterior and an interior: they have also very frequent inosculations or communications with each other throughout their whole course, and there are in many places folds of the internal coats which act as valves, allowing a current in only one direction, namely, towards the heart. These valves, like locks in a canal, divide the column of blood, and lessen the pressure on any one part. In some persons, as they advance in age, owing to a thinning and weakening of the venous coats, the pressure of the blood downwards, as the result of gravitation, so distends the vein as to prevent this mechanical action of the valves, and the disease called varix (from varus, uneven or crooked), is thereby produced. The vein is unnaturally enlarged, and becomes tortuous in its course. For obvious reasons varicose veins are chiefly seen in the legs, especially of aged persons.

There are no valves in the veins of those organs where their presence would interfere with the free passage of the blood. Thus the veins of the lungs have no valves. They bring to the left

Mechanical Effects.

859

side of the heart the blood which has undergone aëration in the lungs.

It is estimated that the capacity of the veins is about three times as great as that of the arteries, and that the velocity of the blood's motion in them is about one-third less than in the arteries. The rate at which the blood moves in the veins gradually increases the nearer it approaches the heart-the sectional area of the venous trunks becoming gradually less. (Kirkes.)

The simple weight of the column of blood in any descending artery is just sufficient to raise the blood through open capillaries to an equal height in the corresponding vein, according to the hydrostatical law, that fluids attain the same level in all communicating vessels; and therefore, as the arch of the aorta rises considerably above the heart, the gravitating pressure of the descending arterial column of blood would be sufficient to lift that in the veins, not only up to the heart, but considerably beyond it. In addition to this influence of gravity on the venous current, the blood is pressed into the arteries, and from them, therefore, towards the veins, with a force from the heart itself, as stated above, of about four pounds to the square inch, or, in other words, as if there were a column of blood eight feet higher than the heart urging the current. It might be expected from the law of equal diffusion of pressure in fluids, that these causes would soon produce a tension in the veins as great as in the arteries: and this does not happen, only because the blood has a ready escape from the veins through the right auricle and ventricle of the heart. Under ordinary circumstances, there can be no greater tension in the veins than what is sufficient to lift the blood to the level of the heart and to overcome the friction.

1095. These facts, then, and others that might be mentioned, prove incontestably that the blood is pressed into the veins from the arteries and capillaries, with a force sufficient to lift it, not only to the heart again, but many feet farther, viz., about as far as it would ascend in a tube rising from the tense arteries themselves. A difficulty appears to have arisen in admitting this explanation from the great disparity observed between the tension in the arteries and in the veins; while it was not considered that this disparity was owing to there being a free passage or outlet from the veins through the heart.

Physiological experiments confirm the view that the contractions of the heart alone supply a sufficient force for the circulation of the