received a considerable share of attention. The results of such experiments, as have been tried, show that this rapidity is much greater than would have been anticipated. Hering, Poisseuille, and Matteucci,' have all experimented on this subject in the following manner. A solution of ferrocyanide of potassium was injected into the right jugular vein of a horse, at the same time that a ligature was placed upon the corresponding vein on the left side, and an opening made in it above the ligature. The blood flowing from the left jugular vein was then received in separate vessels, which were changed every five seconds, and the contents afterward examined. It was thus found that the blood drawn from the first to the twentieth second contained no traces of the ferrocyanide; but that which escaped from the vein at the end of from twenty to twenty-five seconds, showed unmistakable evidence of the presence of the foreign salt. The ferrocyanide of potassium must therefore, during this time, have passed from the point of injection to the right side of the heart, thence to the lungs and through the pulmonary circulation, returned to the heart, passed out again through the arteries to the capillary system of the head and neck, and thence have commenced its returning passage to the right side of the heart, through the jugular vein.

If this experiment were altogether decisive, it would demonstrate that the blood performs the entire round of the circulation in from 20 to 25 seconds. But it is not so conclusive in this respect as might at first be supposed. In reality, it only shows that the solution of the ferrocyanide passes round to the opposite vein during this period, but it does not necessarily follow that the entire blood moves with the same rapidity; since the injected saline solution may diffuse itself through the blood, so as to travel faster than the blood itself. Subsequent experiments of Poisseuille showed, in fact, that other substances injected at the same time may either accelerate or retard the movement of the ferrocyanide. If a little nitrate of potass, for example, were injected together with the ferrocyanide, the latter salt appeared in the blood flowing from the opposite jugular at the end of twenty seconds. A solution of acetate of ammonia, again, shortened the period to eighteen seconds. On the other hand, a little alcohol, injected at the same time, retarded its motion to such a degree, that the ferrocyanide could not be detected till the end of

' Physical Phenomena of Living Beings, Pereira's translation, Philada. ed, 1848, p. 317.

forty to forty-five seconds. These facts show conclusively that the time required for a solution of ferrocyanide of potassium to appear in the opposite jugular vein does not depend altogether on the rate of movement of the blood itself, but is influenced very considerably by the chemical constitution and physical properties of the injected fluid, and its physical relations to the blood and to the walls of the blood vessels. Furthermore, we have already seen that the different ingredients of the blood do not all circulate with the same rapidity. In a microscopic examination, for example, it is evident that the white globules of the blood move much more slowly than the red; and it is very possible that the red globules themselves pass less rapidly from one point to another than those portions of the blood which are entirely fluid.

The truth is, however, that we cannot fix upon any uniform rate which shall express exactly the time required by the entire blood to pass the round of the whole vascular system, and return to a given point. The circulation of the blood, far from being a simple phenomenon, like a current of water through a circular tube, is, on the contrary, extremely complicated in all its anatomical and physiological conditions; and it differs in rapidity, as well as in its physical and chemical phenomena, in different parts of the circulatory apparatus. We have already seen how much the form of the capillary plexus varies in different organs. In some the vascular network is close, in others comparatively open. In some its meshes are circular in shape, in others polygonal, in others rectangular. In some the vessels are arranged in twisted loops, in others they communicate by irregular but abundant inosculations. The mere distance at which an organ is situated from the heart must modify to some extent the time required for its blood to return again to the centre of the circulation. The blood which passes through the coronary arteries, for example, and the capillaries of the heart itself, must be returned to the right auricle in a comparatively short time; while that which is carried by the carotids into the capillary system of the head and neck, to return by the jugulars, will require a longer interval. That, again, which descends by the abdominal aorta and its divisions, to the lower extremities, and which, after circulating through the tissues of the leg and foot, mounts upward through the whole course of the saphena, femoral, iliac and abdominal veins, must be still longer on its way; while that which circulates through the abdominal digestive organs and is then collected by the portal system, to be again dispersed through

the glandular tissue of the liver, requires undoubtedly a longer period still to perform its double capillary circulation. The blood, therefore, arrives at the right side of the heart, from different parts of the body, at successive intervals; and may pass several times through one organ while performing a single circulation through another.

Furthermore, the chemical phenomena taking place in the blood and the tissues vary to a similar extent in different organs. The actions of transformation and decomposition, of nutrition and secretion, of endosmosis and exosmosis, which go on simultaneously throughout the body, are yet extremely varied in their character, and produced a similar variation in the phenomena of the circulation. In one organ the blood loses more fluid than it absorbs; in another it absorbs more than it loses. The venous blood, consequently, has a different composition as it returns from different organs. In the brain and spinal cord it gives up those ingredients necessary for the nutrition of the nervous matter, and absorbs cholesterine and other materials resulting from its waste; in the muscles it loses those substances necessary for the supply of the muscular tissue, and in the bones those which are requisite for the osseous system. In the parotid gland it yields the ingredients of the saliva; in the kidneys, those of the urine. In the intestine it absorbs in large quantity the nutritious elements of the digested food; and in the liver, gives up substances destined finally to produce the bile, at the same time that it absorbs sugar, which has been produced in the hepatic tissue. In the lungs, again, it is the elimination of carbonic acid and the absorption of oxygen that constitute its principal changes. It has been already remarked that the temperature of the blood varies in different veins, according to the peculiar chemical and nutritive changes going on in the organs from which they originate. Its color, even, which is also dependent on the chemical and nutritive actions taking place in the capillaries, varies in a similar manner. In the lungs, it changes from blue to red; in the capillaries of the general system, from red to blue. But its tinge also varies very considerably in different parts of the general circulation. The blood of the hepatic veins is darker than that of the femoral or brachial vein. In the renal veins it is very much. brighter than in the vena cava; and when the circulation through the kidneys is free, the blood returning from them is nearly as red as arterial blood.

We must regard the circulation of the blood, therefore, not as a

simple process, but as made up of many different circulations, going on simultaneously in different organs. It has been customary to

Fig. 101.

extremities. 4. Spleen. 5. Intestine. 6. Kidney. 7. Lower extremities. 8 Liver.

illustrate it, in diagram, by a double circle, or figure of 8, of which the upper arc is used to represent the pulmonary, the lower the general circulation. This, however, gives but a very imperfect idea of the entire circulation, as it really takes place. It would be much more accurately represented by such a diagram as that given in Fig. 101, in which its variations in different parts of the body are indicated in such a manner as to show, in some degree, the complicated character of its phenomena. The circulation is modified in these different parts, not only in its mechanism, but also in its rapidity and quantity, and in the nutritive functions performed by the blood. In one part, it stimulates the nervous centres and the organs of special sense; in others it supplies the fluid secretions, or the ingredients of the solid tissues. One portion, in passing through the digestive apparatus, absorbs the materials requisite for the nourishment of the body; another, in circulating through the lungs, exhales the carbonic acid which it has accumulated elsewhere, and absorbs the oxygen which is afterward transported to distant tissues by the current of arterial blood. The phenomena of the circulation are even liable, as we have already seen, to periodical variations in the same organ; increas

Heart. 2. Lungs. 3. Head and upper ing or diminishing in intensity with the condition of rest or activity of the whole body, or of the particular

organ which is the subject of observation.

CHAPTER XV.

SECRETION.

WE have already seen, in the last chapter, how the elements of the blood are absorbed by the tissues during the capillary circulation, and assimilated by them or converted into their own substance. In this process, the inorganic or saline matters are mostly taken up unchanged, and are merely appropriated by the surrounding parts in particular quantities; while the organic substances are transformed into new compounds, characteristic of the different tissues by which they are assimilated. In this way the various tissues of the body, though they have a different chemical composition from the blood, are nevertheless supplied by it with appropriate ingredients, and their nutrition constantly maintained.

Beside this process, which is known by the name of "assimilation," there is another somewhat similar to it, which takes place in the different glandular organs, known as the process of secretion. It is the object of this function to supply certain fluids, differing in chemical constitution from the blood, which are required to assist in various physical and chemical actions going on in the body. These secreted fluids, or "secretions," as they are called, vary in consistency, density, color, quantity, and reaction. Some of them are thin and watery, like the tears and the perspiration; others are viscid and glutinous, like mucus and the pancreatic fluid. They are alkaline like the saliva, acid like the gastric juice, or neutral like the bile. Each secretion contains water and the inorganic salts of the blood, in varying proportions; and is furthermore distinguished by the presence of some peculiar animal substance which does not exist in the blood, but which is produced by the secreting action of the glandular organ. As the blood circulates through the capillaries of the gland, its watery and saline constituents transude in certain quantities, and are discharged into the excretory duct. At the same time, the glandular cells, which have themselves been nourished by the blood, produce a new substance by the catalytic