There is considerable obscurity respecting the use of the adipose matter. Probably, in the first instance, the se cretion of fat is an operation of that kind which has been denominated excrementitious, where a substance is sepa rated from the blood because it is superfluous, or useless; for it would appear that this fluid contains a less propor tion of carbon and hydrogen than chyle. But, although the primary object of this secretion may be to separate from the blood its excess of carbon and hydrogen, it is ana logous to the other operations of the animal economy to suppose that fat, when once formed, may perform some important secondary purpose. Now it has been stated above, that the inflammable matter which is consumed in the process of respiration is derived from the chyle, and it may be presumed, that, in ordinary cases, the demand and the supply are in due proportion to each other. if, front any cause, the supply be too considerable, we may conjec ture that it contributes to the formation of the adipose matter, which then accumulates in the system ; but if, on the contrary, the demand is increased, and the stomach does not furnish it in adequate quantity, by the usual pro cess of digestion, the fat which had been deposited in its appropriate cells is then taken up by the absorbents, is re turned into the blood, and being carried to the lungs, is there employed in the evolution of animal heat.
Besides the oil or fat, in its more simple form, there are some other secretions of a more compound nature, but which may be referred to this class, as they derive their most characteristic properties from their oily part. Among these we may place milk, a fluid which is secreted by the female after parturition, and which is destined for the first nourishment of the offspring. Although the milk differs considerably in the different classes of animals, yet its prin cipal constituents are always the same, and it is found to consist essentially of albumen, oil, and a peculiar saccha rine matter, the albumen and the oil being united in the form of an emulsion, and the whole dissolved in water. We may suppose that the albumen is immediately derived from the serum of the blood ; but neither the oil nor the sugar previously existed there. The glandular apparatus by which it is produced is large, and well-defined in all its parts, but we are ignorant of the nature of the minute ope rations which are carried on in it, or what relation its dif ferent parts bear to the different constituents of the fluid.
Another substance, still more important to animal exist ence than milk, which may be classed among the oleagi nous secretions, is the matter which composes the brain and nerves. It appears to be formed of a combination of albumen and oil, the albumen being in a state of imperfect coagulation, and the oil approaching to the nature of adi pocire. Except in the absence of saccharine matter, brain may therefore be regarded as bearing a near resemblance to milk ; yet there appears to be no glandular structure appropriated for its formation, nor can we form any con ception of the mode in which it is produced.
As we proceed with our examination of the secretions, we come to those which are more remote from any of the constituents of the blood, and which, while they are the most complicated in their nature, derive their character istic properties from the greatest number of ingredients. This remark applies to the sixth class, the resinous sub stances, of which the principal is the bile. Bile is the pro duct of the liver. a very large gland, and one possessing the most elaborate structure of any of the organs destined for secretion. This fluid has been very frequently made the subject of chemical analysis, and is found to be corn posed of a great number of substances ; but it would ap pear that its specific properties depend upon a peculiar rc sin which it contains. To this resin the colour, taste, and odour of bile are probably owing, as well as its action upon the stomach and bowels. There are certain facts, and some analogies, which would lead us to conjecture that the resin of the bile is immediately derived from the red particles of the blood; and we are farther led to conceive that this conversion is effected by the addition of oxygen to these particles.
Aluch has been written upon the use of the bile, or ra ther upon the relative action which the liver possesses in the system at large. Many of the speculations that have been formed on this subject have been obviously derived from incorrect principles, and we are still unable to advance any thing that may not be liable to objections ; but there are many facts in medicine and pathology which lead to the following conclusion. When venous blood becomes loaded with inflammable matter, which the lungs cannot discharge, and when circumstances arc not favourable to the deposition of fat, the liver is the organ by which this matter, after it has undergone certain changes, is removed from the system. Hence we are to regard bile, like fat, as, in the first instance, an excrementitious substance, al though, in conformity with the usual arrangements of the animal economy, the different organs and functions are so admirably adapted to each other, that a variety of impor tant objects are accomplished by the same operation. Al though, therefore, the primary purpose of the liver be to remove the excess of hydrogen and carbon from the blood, yet the bile appears to exercise a salutary action upon the digestive organs, and perhaps also upon some other parts of the system. Hence we may explain the frequency of bilious disorders in warm climates, where the body, being continually immersed in a high temperature, a sufficient quantity of carbon cannot be discharged from the lungs, and where, from certain causes, there is no tendency to the formation of adipose matter.