same experiment, since the arteries are of firmer structure. Greater care, however, is required in the experiment; for, practically, it is a more difficult task than might be supposed, à priori, to enclose a column of blood in a portion of pulsating artery. Both ligatures must be tied at the same moment, and very firmly, and even then sometimes the occluded part of the vessel is afterwards found empty. But when blood is really entrapped in a piece of artery, the result is admirably effective: not hours in this case, but days will pass, during which the blood remains fluid, for the vessel contracts on the column of blood, and compression is added to occlusion.

It is clear, then, that complete occlusion from air has, under the circumstances named, the opposite effect on coagulation to that of exposing blood to air. But the objection may be urged, that in the experiments above narrated, the blood was enclosed and retained in the bodies of the animals. To remove this objection, I pursued the following course:

I made out of a piece of barometer tubing an instrument, as shown in the opposite drawing.

It consisted of a tube thirty-two inches long, and half an inch in the bore. At its middle the tube was bent in an easy curve, so as to have two equal upright limbs; thus formed, it was fixed on a stand, such as supports a Marsh's bulb apparatus. Prior to using the tube, a piece of ligature cord was passed through it. One end of the cord was affixed to the neck of one of the upright limbs; the other end was left long and free; next, the lower part of the tube was filled with mercury, as shown in the sketch. The further details are given in the succeeding experiments :

After the

EXPERIMENT CLI. Blood enclosed in a Vein under Mercury. I laid bare the jugular vein of a sheep, and tied it firmly at two points: the lower ligature was left long; the upper ligature was taken from the long end of the string running through the tube. ligatures were attached firmly, the portion of vein included within the ligatures was removed altogether from the animal. By drawing out the string attached to the neck of the glass tube, the piece of vein was easily drawn under the mercury, and was there left lying in the bend of the tube. Here it was secured safely by a few turns of the loose

ends of the string

round each neck of the tube: next, the tube was filled with mercury through its entire length, and the whole was left at rest. Meanwhile, an assistant had taken the observation that the blood flowing from the divided vein of the sheep in question coagulated on exposure to the air, temperature 66° Fahr., in one minute and ten seconds.

In

It had been my intention not to disturb the blood buried under the mercury for six hours. But an accident occurred which prevented this intention. moving the tube on its stand to a more convenient and safe position, some unnecessary hurry occurred, the tube got a shake and gave way at the bend, and the mercury escaped. This occurred three-quarters of an hour after the immersion of the vein. The accident was irremediable at the moment. I therefore placed the piece of vein, which luckily was uninjured, in a cup, and punctured it with a lancet. The blood flowed out quite fluid, but set into a firm clot, on exposure to the air, one minute and twenty seconds afterwards.

EXPERIMENT CLII. Blood enclosed in a Vein under Mercury. I laid bare the external jugular vein of a dog, ligatured it as in last experiment, and drew it under mercury in an instrument precisely similar to the one already described. The vein thus placed was left at rest for nine hours: it was then gently removed, placed in a cup, and punctured with a lancet. Its contained blood flowed out in a perfectly fluid condition, but coagulated in four minutes on exposure to the air. A little blood, caught from the vein of this animal directly after the operation to which he had been subjected, coagulated firmly in three minutes on exposure to the air.

EXPERIMENT CLIII. Blood enclosed in a Vein under Mercury. I laid bare the external jugular of a cat, placed an inch of it between two ligatures, removed the ligatured part, and drew it under the mercury as before. This blood was thus left for twenty hours: the vein was then gently withdrawn, placed in a spoon, and punctured. Its contained blood flowed out entirely fluid, and on exposure to the air coagulated firmly in four minutes. Some blood of this same animal, derived also from the jugular vein at the time of operation, coagulated, on exposure to the air, in two minutes and a half.

Writers on natural history, in referring to the medicinal leech, are accustomed to state the fact, that the blood drawn by the leech remains fluid in its body, and serves for its nourishment for long intervals of time. Cuvier remarks, that the blood swallowed by the leech. continues red and without alteration for several weeks; and Griffiths (Animal Kingdom, part 35, p. 129), says that in the leech the process of digestion is extremely slow, and that a single meal of blood will suffice for many months, during all which period "so much of the blood as remains undigested in the stomach continues in the fluid state and as if just taken in, notwithstanding the vast difference in the heat of the body of a mammiferous animal and that of a leech."

It is clear that in writing this passage, Mr. Griffiths laboured under the erroneous idea that this fluidity of blood is due to the presence of heat. The fact, however, about the leech is of interest, and the study of it has some points bearing on coagulation. is allowed to engorge itself, and is then removed and emptied of its blood by firm pressure with the finger

If a leech

and thumb, the blood thus extracted will be found quite fluid, and its power of coagulation exceedingly feeble, and in some cases nil. I have seen slight coagulation in the blood thus obtained from leeches, but I have also seen the blood so obtained perfectly uncoaguable. I have observed also that leech-bites continue to bleed for a much longer period than do wounds from a lancet point made in the same individual at the same time; while the first blood which flows from the bite after the removal of the leech continues uncoagulable for a very long time, and is even in some cases also uncoagulable. This fact accounts for the difficulty which not unfrequently occurs in the stopping of the bleeding from leech-bites. It is not until the blood has flowed away for some minutes, that a coagulable current is supplied.

The cause of this simple but striking phenomenon is not very easily explained. The first inference on my mind was, that in the body of the leech the blood meets with some solvent secretion which holds it permanently fluid, or else that in the mechanical act of abstracting the blood some change occurs, some kind of defibrination. To follow these questions, I made a few simple experiments on leech-drawn blood.

EXPERIMENT CLIV. Observation on Leech-drawn Blood. I laid a leech on the arm of a gentleman who was in perfect health. The leech filled itself with blood, and fell off of its own accord. As soon as it removed itself, I emptied it of its contained blood by traction. The blood was dark and entirely fluid. This blood remained fluid on exposure to the air (temperature 65°) in a shallow cup, for nine hours; after this it became a little thicker and more glutinous, but it never underwent the proper process of coagulation.