It is farther important to attend to the ele mentary composition of fat. Each variety of fat consists of carbon, hydrogen, and oxygen ; and a few, as hog's lard, blubber, nut oil, and almond oil, contain a small trace of azote. The proportion of the carbon is greatest and varies in general from 7-10ths to 4-5ths of the whole. The proportion of hydrogen varies from 1-10th to 1-5th. That of oxygen varies from four or five parts in the hundred to 12 and 13. It appears, therefore, that fat and each of its constituent principles are a highly carbonaceous animal substance.
Little doubt can be entertained that animal fat is the result of a process of secretion. But it is no easy matter to determine the mode in which this is effected. Previous to the time of Malpighi it was a very general opinion that the blood exuding from the vessels was con verted into adipose matter. This fancy was refuted by Malpighi, who, departing, however, from strict observation, imagined a set of ducts, (ductus adiposi) issuing from glands, in which he conceived the fat to be elaborated and pre pared. To this fancy he appears to have been led by his study of the lymphatic glands, and inability to comprehend how the process of secretion could be accomplished by arteries only. The doctrine, though embraced by Perrault, and Hartsoecher, was over thrown by the strong arguments which Ruysch deduced from his injections; and Malpighi afterwards acknowledged its weakness and re nounced it. In short, neither the glands nor the ducts of the adipose membrane have ever been seen, unless we admit the testimony of the Members of the Parisian Academy, who state that they saw them in the civet cat, and to this we must oppose the fact that Morgagni, by anatomical evidence, disproved their ex istence. Winslow, though willing to adopt the notion of Malpighi, admits, nevertheless, that the particular organ, by which the fat is sepa rated from the blood, is unknown. Haller, on the contrary, aware of the permeability of the arteries, and inferring from the phenomena of injections either of watery liquors or melted tallow, their direct communication with the cells of the adipose tissue, and trusting to the testimony of Malpighi, Ruysch, Glisson, and Morgagni, that fat exists in the arterial blood, saw no difficulty in the doctrine of secretion, or rather of a process of separation ; and upon much the same grounds is this opinion adopted by Portal. Bichat, again, contends that no fat can be recognized in the arterial blood, and justly adduces the fact, that none can be dis tinguished in blood drawn from the temporal artery. To the accuracy of this fact I can bear direct testimony, having repeatedly ex amined with the view of recognizing the bully coat, and detecting oily particles, blood, which I had drawn from this vessel,—the latter sub stance invariably without success. In wounds in the human body during life, and living animals, oily particles may be seen floating on the surface of the blood ; but these, it may be said, proceed from the division of the adipose vesicles; and hence it has been inferred that the arterial blood contains no adipose or olea ginous matter.
It may be doubted, however, whether facts of this kind are adequate to prove whether adipose or oily matter does not naturally exist in the blood, and both from the experiments of Chevreul, and those of Lecanu and Boudet it appears that small quantities of adipose or puriloid matter may be obtained from this fluid. M. Chevreul, for example, shows that fatty matter may be obtained from the fibrine of arterial blood ; and from a series of elabo rate and accurate experiments, estimates the quantity of fatty matter in fibrine at from four to five per cent.* Lecanu and Boudet have also recently shown that crystals of pearly coloured matter having the characters of an adipose substance exist in, and may be ob tained in small proportion from the serum of the blood.1- These inferences apply, according to the authors, to blood in its healthy state.
In certain states of the system the blood drawn from the veins has presented serum of an opaque or milky appearance, and which has been proved to depend on the of adipose or oleaginous matter. Thus, indepen dent of opaque or milky serum noticed by Schenke, Tulpius, Morgagni, and others, Ilewson and several cotemporary observers remarked instances of opacity and milkiness of the serum of the blood, and from ocular inspection as well as experiment and obser vation, inferred that these appearances arose from the presence of oil in the blood or its serum. Soon after Dr. Gregory, in his Con spectus, or View of the Institutions of Medicine, was led to infer apparently from the fact stated by newson, that in persons in whom the serum was opaque or milky, this depends on the presence of fat which is undergoing ab sorption, or resumption into the system. This representation, however, was entirely conjec tural; and no direct proof of the fact that oil does exist in certain states in the venous blood was given till Dr. Traill, in 1821 and 1823, furnished accurate chemical evidence on the point. The inferences of Dr. Traill have been since confirmed by the experiments of Dr. Christison, who found that milky serum con tains oleaginous or adipose matter, consisting of the two adipose principles elaine and The general conclusions, therefore, that may be deduced from the now stated are that in the healthy state adipose matter in small proportion exists in the fibrine of the blood, and in a still smaller portion in the serum ; and that in certain morbid conditions of the system, in which there is any process of mis nutrition or paratrophia, oily matter in con siderable quantity may be found in the blood, either in consequence of absorption or non deposition.