The seventh and last class of secretions are the saline. The most important of these is phosphoric acid, which, when united with lime, forms the earth of bones; the same acid also forms a considerable proportion of the salts of the urine. A curious question, and one that is of difficult so lution, here presents itself ; whether the saline substances that exist in different parts of the body are formed there, or whether they are not received into the stomach, and carried by the absorbents into the blood. This investiga tion is intimately connected with many important points in the physiology of some of the inferior animals. A large part of the whole substance of several of the molluscs and the crustacea consists of calcareous matter ; and it appears that many of the large masses of limestone which occur in different parts of the world, have originated from the de composition of immense numbers of these animals. What is the origin of this lime ? Did it exist previous to the for mation of these animals, and did they receive it into their system, and organize its particles, so as to form their shells and crusts, and afterwards, by their destruction, compose the masses in which it is now found ? or have their diges tive and secreting organs the power of actually generating lime ? Each of these opinions has had its advocates, and experiments have been advanced on both sides ; but we conceive they have not been of that unequivocal nature as to warrant our drawing any decisive conclusion from them. Upon the whole, however, the result of the experiments that would seem the least exceptionable are in favour of the generation of lime by the powers of the living body ; but it must be acknowledged that this is a conclusion which would involve many suppositions that are at vari.
ante with our ordinary ideas respecting the economy of nature.
After this attempt to form a classification of the secre tions, we must proceed to consider the nature of the ope ration by which they are produced. The opinions that have been entertained on this subject are very numerous ; but they may be reduced to five distinct hypotheses ; that of the older chemists, who ascribed secretion to fermenta tion; that of the mechanicians, according to whom the substances are separated front the blood by filtration ; the doctrine of the Animists, who conceive it to be a sufficient explanation to ascribe it. to the immediate operation of the vital principle ; that of the modern chemists, who suppose that the elements of the secretions exist in the blood, and that, in the passage of this fluid, or some of its component parts, through the glands, new affinities are called into ac tion, which produce new combinations ; and lastly, the hy pothesis which attributes secretion to the influence of the nerves.
The doctrine of ferments, as the cause of secretion, was advanced by Van Hclmont ; it was farther matured by Syl vius, and received various modifications from Willis, and other eminent physiologists of that period. Each gland was supposed to possess a peculiar species of fermentation, which assimilated to its own nature the blood that passed through it, as is the ease in the formation of vinegar. Re specting this hypothesis it will be sufficient to remark, that the resemblance between secretion and fermentation is ra ther apparent than real ; and that, since we are become better acquainted with the circumstances that are neces sary for the production of this latter process, and with the substances that are generated by it, we find the phenomena of the two operations to be very different from each other.
When the chemical physiology began to be exploded, and mechanical doctrines were had recourse to, the hypo thesis of filtration was adopted. It was supposed that the secretions were already formed in the blood, and that, when portions of this fluid were carried to the different glands, the respective secretions were separated from it as through sieves or filtres. In order to explain how so many differ ent substances could be separated from the same fluid merely by filtration, two conjectures were formed by the earlier physiologists. The first to which we refer is that of Descartes, who supposed that the particles of the se creted fluid were of a particular shape, for example, circu lar, triangular, four-sided, &c. and that the gland which separated them had pores of a similar form. Every one in the present day must regard this opinion as completely whimsical, and as altogether without any support of facts ; it is obviously less philosophical than the hypothesis which was opposed to it, and which appears to have originated with Leibnitz. He compared the action of the gland to that of a filtre which had its pores saturated with a parti cular fluid, so as not to permit any other to pass through them, in the same manner as paper moistened with water will not permit oil to pass, and vice versa. But this ex planation, although ingenious, appears to be purely hypo thetical; and it proceeds upon the supposition that the se cretions exist ready formed in the blood, of which we have )o evidence, or rather a strong presumption that it is not the case.
The doctrine of Haller on the subject of secretion is fun damentally that of filtration, although much modified, and considerably more consonant to our modern notions than that either of Descartes or Leibnitz. As it is merely me chanical, it cannot be admitted ; but we must acknowledge. that it displays his usual caution in drawing conclusions where the premises are of doubtful authority. He pro ceeds upon the supposition that the secretions arc all ready formed in the blood, and he then inquires what arc the cir cumstances that are likely to effect their separation from it ? He points out seven. 1. A difference in the nature of the blood itself; 2. Its velocity as affected by the size of the vessel; 3. The transmission of the fluid from one ves sel to another of a different size ; 4. The angle at which the secreting vessel passes off from the artery ; 5. The winding course of the vessel ; 6. Its density ; and, lastly, the structure of the excretory duct. Haller himself prin cipally insists upon the different sizes of the vessels, and the different velocities of the fluids; and conceives that, in proportion to the existence of these causes, the four classes of secretions which he points out will be produced ; and we may go so far as to allow, that these, as well as the other mechanical causes enumerated above, may consider ably influence the action of the glands, but we are of opi nion that they are alone quite inadequate to their produc tion.