means, to keep a column of blood in steady circular play. I should waste time by describing the many mechanical difficulties which I met with in this simple experiment. The difficulties arose mainly from the impossibility of making valves that should exactly imitate those in the body, and should play as well. In one apparatus, which was worked by a double piston, I used metallic valves, and succeeded in sustaining a water current through it. With blood, however, it would not answer at all; for the fibrin adhered to the valves at once, and blocked up the tubes. At last I thought of the single heart of fishes, and took this in some measure for a pattern, with the difference of inventing a mode by which valves within the tubes might be dispensed with. In the end, the apparatus represented

here was devised, and answered well. It was made

up of two bags (cc) of thin vulcanised India rubber. The bags were each capable of holding an ounce and a half of fluid. From each bag ran out two tubes of the same material. These tubes were three inches in length, and were slipped on to two curved glass tubes (B B B B), so as to complete a circuit with the bags, which were opposite to each other in the semi-circumference on each side. The circuit thus made was fixed on a strong wooden stand. A light beam (D) extended across the table from one bag to the other; each end of the beam being provided with a kind of cushion (E E) adapted to the bag. The beam turned vertically on a pivot (P) in the centre, and was provided with a handle at each end for assisting the

movements.

By simple spring valves (GG) placed at opposite positions at the termination of each of the arms of the beam, the passage of the fluid was kept in one direction; and, by the movement of the beam first on to one bag and then on to the other, the onward motion was secured.

[When the apparatus was to be used, blood freshly drawn was poured into the circuit. In my first instrument, which was sent in to Guy's Hospital, and which was hastily made, the blood was introduced by slipping off one of the India rubber tubes from the glass tube, and then rejoining them. In this way Experiment CLXIX was performed. Afterwards, I improved the apparatus by fitting to it at the upper part a funnel (A) with a stopcock, which stood upwards at a right angle with the tube. When the apparatus was filled, the stop-cock was turned, and the motion of the beam was commenced. By a little care and dexterity, the process of filling was performed in half a minute.]

When freshly drawn blood was introduced so as to fill

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the apparatus in the way described above, and was set in motion in the circuit by the action of the levers, it never coagulated so as to stop the current; but this ultimate result was due chiefly to the gradual separation of the fibrin, which deposited on the inner surfaces of the India rubber bags. At the same time, steady circular motion of this kind, in a closed circuit having the least possible tendency at any point to retard the current, shewed a marked effect in delaying coagulation: for I found that the separation of fibrin in the apparatus was not immediate, since blood, drawn in a fluid state from the apparatus after it had been kept in motion ten minutes, and in one instance a quarter of an hour, coagulated on removal. But it has an interesting bearing on some points in pathology, to know that, after a longer lapse of time, the fibrin commenced to be deposited in the interior of the bags, whence it could be washed in fine filamentous threads. After this separation, the blood of course remained permanently fluid. The blood-corpuscles were uninjured.

EXPERIMENT CLXIX. Blood in Circular Motion. The apparatus already named was filled with blood immediately after it had been drawn from a free wound in the neck of a heifer. The artificial circulation was at once set up, and was steadily continued without interruption for forty minutes. As the blood was being propelled from one sac to the other, a distinct pulsation could be felt in the elastic tubes. By gently compressing these, the fluid character of the blood could be ascertained. After the motion had been kept up for ten minutes, a little blood (about half a drachm) was allowed to escape, by slightly raising one of the India rubber tubes from over the glass tube. This blood,

flowing out in a fluid state, and of a bright red colour, coagulated in three minutes. Ten minutes later, a little more blood, drawn in the same way, shewed scarcely any tendency to coagulate; and, at the end of the experiment, the whole of the blood, on being received from the apparatus into a vessel, was found defibrinated and permanently fluid. I afterwards washed out, from the bags of the apparatus, the fibrinous filaments, which seemed to have been separated exclusively in the bags, and were perfectly free from any red colouring matter.

[EXPERIMENT CLXX. Blood in Circular Motion. The apparatus already named, as modified in the description, was filled with blood freshly drawn from an ox, and was immediately set in motion. After the motion had been continued fifteen minutes, a little blood was withdrawn, and coagulated on exposure to the air at rest. Some more blood, drawn after a continuance of the motion for fifteen minutes longer, shewed no coagulating power, being defibrinated. The fibrin, as in the previous case, was deposited in filamentous layers in the India rubber bags. This experiment was repeated many times with the same result.

The series of experiments narrated in this chapter supply, in brief, the following facts.

1. Temperature. The power of coagulation of blood is reduced in proportion as the blood is absolutely brought down in temperature. Such reduction of temperature may be carried to freezing point: the frozen blood remains uncoagulated until the application of heat; but, on the blood being thawed and exposed to a higher temperature, the process of coagulation manifests itself. Conversely, coagulation is quickened in proportion as

the blood is raised in temperature above its natural standard, near to the degree at which the albumen coagulates. If two specimens of blood, taken at the same time from the same animal, be placed in such positions. as to insure an absolute difference in temperature, there is an absolute difference in the periods of coagulation.

2. Addition of Water to Blood. The addition of distilled water to blood, in the proportion of equal parts of each liquid, does not materially influence the period of coagulation, unless the temperature of the water added be raised to that of the blood; under which circumstance, the coagulation is quickened. The addition of distilled water in excess to blood retards coagulation; and, when carried to an extreme, causes a simple separation of fibrin in a pale flocculent or gelatinous form.

3. Addition of Dense Fluids to Blood. The admixture of blood with fluids at or above its own density materially retards coagulation, and in some instances gives rise to the separation of fibrin in an upper layer or coat.

4. Exposure to or Exclusion from Air. Free exposure of blood to air quickens coagulation. The vacuum also quickens coagulation. Exclusion from air, whether in a vessel of an animal, or in an inorganic structure, retards coagulation; and when to perfect occlusion pressure added, the retardation is sustained.

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5. Motion. Agitation of blood in the open air quickens coagulation. Motion in a closed vessel retards coagulation. Blood received into a closed circuit made up of inorganic materials, and constructed so as to imitate closely in physical respects the circulatory system, may be kept in motion in such circuit for a brief period without coagulating; but ultimately the fibrin is deposited on the expanded parts of the circuit.]