The Philosophy of Health; Volume 2 (of 2) / or, an exposition of the physical and mental constitution of man

by Southwood Smith

912. Thus hybernating animals, which pass many months without taking food, accumulate a store of fat before they fall into the state of torpor. Marmots and dormice subsist on this store during the winter, and hence, when spring awakens them from their torpor, they are always in a state of extreme emaciation. Birds and other animals which live on food procured with difficulty in the winter, become unusually fat in the autumn.

913. During fever and other acute diseases, when little food is received, and still less converted into chyle, the extreme emaciation which the body undergoes is owing partly to the disappearance of the fat, which is taken up by the absorbents and carried into the blood, in order to compensate for the deficiency of nutrient matter supplied by the digestive organs.

914. The chief depositories of the fat are those intersticial spaces of the body in which a certain quantity of soft but tenaceous substance is required to obviate pressure or to preserve symmetry. A large quantity is also placed immediately beneath the skin; in the interstices of muscles; along the course of blood-vessels and nerves; in the omentum, where it is spread like a covering over the viscera of the abdomen (fig. [CLXX]. 7); in the mesentery and around the kidneys.

915. Fat is a bad conductor of heat; consequently the layer which is spread over the external surface immediately beneath the skin, and that which is collected in the interior of the omentum, must be useful in preserving the heat of the body. Fat persons bear cold better than lean persons. Animals which inhabit the northern climates, and the fishes of the frozen seas, are enveloped in prodigious quantities of fat. Where the accumulation of this substance would produce deformity or interfere with function, as about the joints, in the eyelids, within the skull, not a particle is ever deposited. About the joints it would impede motion; in the eyelids it would render the face hideous and obstruct vision; and within the skull, a cavity completely filled with the brain, an organ impatient of the slightest pressure, had a substance been placed, the quantity of which is liable to be suddenly trebled or quadrupled, changes in the system which now produce no inconvenience would have been fatal. Thus, while provision is made at once to exonerate the system from too great a load of nourishment, and to lay up the superfluous matter, as in a magazine, to be ready for future use, the most extreme care is taken to deposit the store in safe and convenient situations.

916. The excretory organs and processes, hitherto considered, have for their object the removal from the blood of its superfluous carbon and hydrogen; the element peculiar to the animal body, azote, is eliminated by the kidneys, glandular organs which possess a highly complex structure.

917. But besides the removal of the superfluous azote, the fluid secreted by the kidneys would appear to be a general outlet for whatever is not required in the system, and for the removal of which no specific apparatus is provided. Chemical analysis shows that, in different states of the system, the following substances are contained in this fluid:—water, free phosphoric acid, phosphate of lime, phosphate of magnesia, floric acid, uric acid, benzoic acid, lactic acid, urea, gelatin, albumen, lactate of ammonia, sulphate of potash, sulphate of soda, fluate of lime, muriate of soda, phosphate of soda, phosphate of ammonia, sulphur, and silex.

918. This catalogue itself suggests the idea that when any matter employed in carrying on the functions is in excess, or when it has become decayed, or is decomposed and is not eliminated by any other excretory process, it is taken up by the absorbents, poured into the veins, and so conveyed in the course of the circulation to the kidneys, by which organs it is separated from the blood, and thence by an appropriate apparatus carried out of the system.

919. The specific matter secreted by the kidneys is that termed urea; a substance of a resinous nature, highly animalized. One character by which the animal is distinguished from the plant is its locomotion. The organ by which the animal is rendered capable of performing the function of locomotion is muscle or flesh. The basis of muscle is fibrin, and the basis of fibrin azote. There must be in the animal body an abundant supply of fibrin, and consequently a proportionate abundance of azote. Azote is introduced into the system partly by the food and partly by the lungs. That there may be a sufficiency for all occasions, more is introduced than is necessary on ordinary occasions, and a special outlet is established for the excess through the kidneys.

920. Organs appropriated to the removal of substances from the blood, capable of becoming deleterious by their accumulation, generally in a state of health perform their office so perfectly that the matters which it is their part to excrete are eliminated almost as quickly as they enter the blood, so that they are seldom present in the circulating fluid in sufficient quantity to be detected by the most delicate chemical tests. But by the removal of the excretory organ, or by the suppression of its function, the excretory matter accumulates in the blood, and is then readily detected. A decisive experiment disclosed that this is the case with regard to urea. The kidneys were removed from a living animal. The operation did not appear to be productive of material injury for some time; but at length symptoms denoting the presence of a poison in the blood arose, and the animal died. The blood was carefully examined after death. It was found to contain a much larger quantity than ordinary of the peculiar animal substance which enters into the composition of the serosity of the blood (225). On subjecting this substance to the action of various re-agents, and also on reducing it to its ultimate elements, it was found to resemble urea; to be, in fact, nearly identical with urea as contained in the urine. From this experiment it became manifest that the source of the urea is the serosity of the blood. It is probable that the chief office of the kidney is to separate the urea from the other ingredients of the blood, and to convey it to the organs which are destined to carry it out of the body.

921. It is estimated that about a thousand ounces of blood pass through the kidneys in the space of an hour; itself a sufficient indication of the importance of the excretion performed by this organ, and an adequate source of the matter actually excreted, although, under ordinary circumstances, distributed through the circulating mass in quantities so minute as to be almost inappreciable.