which the blood is oxidized; and, within natural orders, it has a relation to the activity of the animal. For example, that of the swallow is 111°, hen 109°, gull 100°; among mammals, squirrel 105°, cat 101°, dog 99°, man 98°. The animals called cold-blooded are only relatively so, for fishes and reptiles have a temperature somewhat above that of the water or air in which they live. Even the lower creatures are slightly warmer than the surrounding medium.
The weight of the blood, which is always greater than that of water, depends, of course, upon the amount of solid matter and the abundance of the corpuscles. In man, the red corpuscles constitute one third to a little less than one half the mass of the blood. The blood of birds has the largest proportion; and it appears that the temperature bears a relation to the amount of solid matter.
The amount of blood is greater in warm-blooded animals; and the proportion of blood to the size of the body increases with the size. The blood of man is by weight about one thirteenth the weight of the body. The dog has blood equal to one fifteenth its body-weight; rabbit, one eighteenth; cat, one twenty-first. The amount of blood in the elephant and the whale has not been determined; but the heart of the whale is three feet in diameter.
The operation of transfusing the blood of a living animal into the vessels of another, or of one that is dying, was known in ancient times, and has been practiced at intervals for the last three hundred years. Extravagant hopes concerning it were formerly entertained. It was believed that diseases might be cured, impaired reason restored, old age deferred, and even the dead returned to life. In late years the eminent Brown-Séquard states that a dead dog was by this means restored to life for twelve hours; but the experiment has never been confirmed, and doubtless the animal was not dead, as supposed. It is also stated that a maniac was restored to reason by the blood of a calf.
In modern medical science, the transfusion of blood has become a well-recognized operation for cases of exhaustion from simple loss of blood. For this it is frequently practiced, and with success in the majority of cases. For general weakness and disease it has sometimes been used, but has not proved reliable.
The amount of blood used in transfusion is usually a very few ounces, sometimes only one or two drachms—rarely ten or more ounces; a small quantity is safer. The blood of a different species of animal is considered dangerous when used in large quantity. Venous blood is preferred, and may or may not be defibrinated.
Pure milk has been successfully used instead of blood; and even artificial mixtures are employed. Richardson kept a monkey alive for several weeks by a daily injection of an artificial blood.
The veins of the extremities are generally selected for the operation, they being less likely to admit air, which might be fatal by causing coagulation.