The second class of secretions, the albuminous, are more numerous and better defined than the aqueous. All the cavities of the body, which have no natural communi cation with the atmosphere, as those of the thorax and ab domen, the ventricles of the brain, the pericardium, and all the parts of the cellular texture, are lined with what is denominated a serous membrane, which secretes a fluid that, except in the proportion of its ingredients, seems to be exactly similar to the serum of the blood. These con stitute the albuminous secretions, and, like serum, possess the characteristic property of being coagulated by heat and various chemical re-agents. In diseased states of the serous membranes, these fluids are often disposed to ac cumulate in their respective cavities, and sometimes their quality as well as their quantity is changed. In some of these secretions, as in that which is found in the cavities of the brain, the quantity of albumen is so small, that we are scarcely able to detect it; the liquor pericardii contains more animal matter, while the dropsical effu siot,s that are poured into the cavity of the abdomen dif fer but little from the serum of the blood. The most mi nute anatomicacl investigation has not been able to detect any appropriate organ for the production of this class of fluids ; and, in consideration of their nature, it may be doubted whether these, as well as the aqueous secretions, are not more properly to be regarded as transudations.
The third class of substances, the mucous, are more strictly entitled to the appellation of secretions, than either the aqueous or the albuminous, as they derive their most characteristic physical and chemical properties from an ingredient which does exist in the blood. They are distinguished by their viscidity, or the capability of be ing drawn out into threads, and by being with difficulty dissolved in water, although they are always combined with a large quantity of it. In some respects they are de cidedly different front the serum of the blood, and the im mediate cause of this difference would appear to be, that they contain a substance which resembles albumen in the coagulated state. The mucous secretions differ from the albuminous in their seat, as well as in their properties; for while the !atter are found on the surface of the mem branes which line the close cavities of the body, the mu cous are poured out over the surfaces which comunicate with the atmosphere, as those of the mouth, the nose, the oesophagus, the stomach, and the whole course of the ali mentary canal, the trachea, and its ramifications through the interior of the lungs. A glandular apparatus may be generally detected in these parts, although we know little of its minute structure. In some cases, as in that or the saliva, the gland that secretes the mucous is unusually large, and possessed of all the parts which enter into the composition of the most perfect glands. The bladder
and the urinary passages are lined with membranes which secrete a mucous fluid ; and it is probable that the tears and the synovia also belong to this class of substances.
The fourth class of secretions which we enumerated are the gelatinous. We have had occasion to give an account of the-properties of jelly, the most characteristic of which is its solution by heat, and concretion by cold, producing the phenomena to which the term gelatinization has been appropriated. Jelly was formerly supposed to be one of the ingredients in the blood ; but this we now know is not the case. There are some circumstances which would lead us to suspect that albumen is converted into jelly by the addition of oxygen ; and we may conceive that some interchange of the elements of the blood may take place, so as to produce this conversion. No apparatus has been detected by which jelly is produced ; and it may, upon the whole, appear more probable, that it is actually formed in the parts where it is deposited, than that it is separated by the medium of a gland. On this point, however, we have scarcely any thing to guide our opinion, either of fact or of analogy. Jelly does not appear to form a component part of any of the animal fluids, but it enters largely into the composition of many of the solids, especially of the parts of young animals.
The fifth class of secreted substances, the oleaginous, are removed still farther from the natural state of any of the constituents of the blood. All animals that possess a temperature much superior to that of the atmosphere are furnished with a quantity of oily matter, which varies in its consistence from the fluid state, as it exists in the whale, to that of a soft solid, as in suet and tallow, while it agrees in its other chemical and physical characters. Oil differs from blood in containing no azote, and a larger proportion of hydrogen ; so that the secretion of oil must consist in separating from the mass a substance which has a basis of carbon, with an excess of hydrogen, and a small quantity of oxygen. We are entirely ignorant of the apparatus by which these secretions are produced, but they appear to be formed with great facility, because there is no animal substance which is so rapidly generated as fat ; and we not only find that fat is very readily deposited, but that it is more quickly removed than any other of the components of the body. Whenever the absorbents act more power fully than natural, or rather, where their action is not coun terbalanccd by that of the nutritive arteries, the fat first disappears, and this is equally the case whether it be the effect of disease or of old age.