To account, however, for the secretion of adipose matter, it is not absolutely requisite to prove that oleaginous or adipose matter exists in the circulating fluid. Even were it ascer tained that oil or adipose matter does not exist, or cannot be detected in any of the elements of healthy blood, the fact would not form a stronger argument against its formation from that fluid, than in the case of several other principles which enter into the composition of the animal tissues, and which nevertheless do not exist in the blood. Thus neither gela tine, which exists abundantly in skin, tendon, cartilage, ligament, and bone,—nor osmazome, which is found in muscle, are contained in healthy blood. But we know that the chemical element of these substances exist in the blood, and we farther know that gelatine consists very nearly of the same chemical elements as albu men ; and we must infer, therefore, that it is the faculty of the living tissues or vessels to arrange these elements in that manner and proportion in which they may constitute re spectively gelatine and osmazome. The same reasoning may be applied to explain the for 'mation of fat in the adipose tissue. Its ele ments already exist in the blood, and the living agency of the tissue seems all that is requisite to effect its deposition. Its composition and history would also show that it is a secreted product which consists of superfluous chemical elements not required in the formation of the albuminous and gelatinous tissues.
On this subject the interesting experiments of Berard and Dobereinert may, perhaps, fur-. nish some intelligible means of illustration. The former chemist found that by mixing one measure of carbonic acid, ten measures of carburetted hydrogen, and twenty of hydrogen, and transmitting the mixture through a red-hot tube, he procured artificially several white crystals which were insoluble in water, soluble in alcohol, and fusible by heat into an oily fluid. The latter chemist prepared a similar substance from a mixture of coal-gas and aque ous vapour.
It may therefore be inferred that while ani mal fat is chiefly a combination of bicarbonated hydrogen with oxygen, or, in other words, a highly carburetted hydrate of oxygen, and con tains either little or no azote, it is the animal substance which makes the nearest approach in chemical constitution to the vegetable prin ciples. So close, indeed, is this approxima tion that Raspail thinks it may be in this re pect compared with starch; and as the different forms of fecula are prepared by the vegetable tissues fOr the nutritious stores of the vegetable during the process of development, he ob serves that, in like manner, fat is deposited whenever the nutritious function is in excess in the animal organs.
It was a singular fancy of Fourcroy that the deposition of fat in animal bodies was in tended as a sort of vent for the superfluous and unnecessary proportion of hydrogen, since the idea is at Variance with chemical facts; and it is not less singular that such a hypothesis should receive any countenance from Blumen bach. Carbon is the principle which predo
minates most largely in fat ; and if any atten tion is to be given to such views, the adipose tissue ought to be regarded as the outlet for superfluous carbonaceous matter, or at least carbonaceous matter in a much larger pro portion than hydrogen and oxygen. The pro per physiological view, however, of this ques tion appears to be,—that as the tissues of the animal body consist chiefly of carbon, hy drogen, oxygen, and azote united in variable proportions, and as most of these tissues either contain or seem to require azote, the adipose appears to be destined to receive whatever carbon, hydrogen, and oxygen, are not re quired to be united with the azote, in the forma tion of the albuminous, the gelatinous, or the albumino-gelatinous tissues.
On the mechanism of the deposition of fat we possess no exact information. But various facts may tend to throw some light on the cir cumstances under which it takes place, and the history of the state of the adipose tissue at different periods of life is instructive.
In the fcetus the adipose tissue contains a sort of whitish, solid, granular matter, which resembles adipocere rather than genuine fat.
Thus it is less oleaginous, and more brittle and friable than true fat. In the infant this layer continues the same in quantity, but a little more oleaginous, till the period at which the individual begins to exert the muscles of loco motion. The fat then rapidly diminishes in quantity, and after the child has begun to walk and run, the fat has almost entirely dis appeared from most parts of the adipose tissue, except the orbits, cheeks, neck, buttocks and the flexures of the joints ; but even in these regions it is much less abundant and much more consistent.
The marrow presents similar changes. The bones of the foetus are void of a distinct me dullary canal, and present merely a reddish, homogeneous, vascular pulp, somewhat con sistent, but presenting soft compressible por tions. This state continues some time after birth. As the individual passes from infancy to childhood, the interior of the bone is formed into cancelli, adipose or oleaginous matter is deposited in the intra-osseous tissue within the cancelli, and as the vessels of the medullary membrane gradually mould the medullary canal, this oleaginous matter is most abun dantly deposited in the interior of the cylin drical bones. The marrow, however, is much less oleaginous, and more like a pulpy paste than it is in the adult.