The obvious objection to this hypothesis was the difficulty in conceiving how mere filtration could separate so many substances from one fluid; but in order to meet this objection two speculations were proposed, for one of which it seems that we are indebted to Descartes, and the other to Leibnitz. The first of these philosophers proposed the whimsical and perfectly gratuitous supposition, that the particles of the various secreted substances were of different figures, and that the pores of the glands possessed the same figures, each gland therefore allowing those particles pass through it which possessed the same figures with its own pores. The hypothesis of Leibnitz, although equally unsupported by facts, is less palpably absurd; he compared the glands to filtres which had their pores saturated with their own peculiar substance, so as to admit of this substance alone passing through them to the exclusion of all the others, in the same manner as a paper saturated with oil prevents the passage of water, and vice versa."

3

4

3 De homine, p. 18. § 11, et de form. fœtus, p. 212. § 25. + Quesnay adops the hypothesis of filtres, but adds to it the operation of the nervous influence; he supposes that secretion is accomplished by the joint action of "the sensibility of the filtres" and the acrimony of the fluids; Econ. Anim. t. 3. p. 437, 8; the section is entitled, "Affinity of the Secretory Organs with the juice which filtre through them." Haller gives us the names of various eminent anatomists and physiologists who adopted the one or the other of these hypotheses. He is led to make the following observation, of the truth of which these pages afford us so many examples. "Sæpius monui, infelicibus exemplis expertus, raro eam esse hominum felici

As our knowledge of the nature of the secretions was gradually advanced, and when the improved spirit of philosophical inquiry taught us to reject such purely conjectural speculations, it was found necessary to examine a little more minutely into the circumstances which might be supposed to operate in the production of the secretion, and to compare these with the actual state and condition of the secretory organs. Still, however, the mechanical doctrines continued to prevail among the most enlightened physiologists, and both Boerhaave and Haller men

tatem, ut vera sint, quæ facile et sponte quasi menti se offerunt." El. Phys. vii. 3. 29. Some judicious remarks on the mechanical hypothesis of secretion are contained in a paper of Winslow's; Mem. Acad. Scien. 1711, p. 241, et seq.

5 Prælect. § 253, cum notis.

❝ Haller's doctrine respecting'secretion is contained in the first 27 paragraphs of the 3d section of his 7th book; it may be asserted that no part of his great work displays in a higher degree his extensive information and correct judgment. It is to be observed that Haller laid it down as the foundation of his hypothesis, that the secretions all exist perfectly formed, or nearly so, in the blood, El. Phys. vii. 1. 8. and that he regarded the glands in no other light than as sieves or strainers to carry off their appropriate fluids, vii. 2. 1. et alibi. His great argument is the facility with which metastases take place, which, as he supposes, proves that the gland cannot form the substance which it discharges; he particularly notices the fact that urine is found in the fluids after the destruction of the kidney, vii. 1.9. Although not directly applicable to the function of secretion, yet I am induced to refer my readers to a paper of Balguy's, on the mode of ascertaining the doses of certain medicines, Ed. Med. Ess. v. iv. p. 33, et seq., as a curious example of the length to which the mechanical physiologists carried their doctrines.

tained opinions respecting secretion, which are es sentially of this description, although reduced into a much more rational and tangible form. Haller displays his usual candour and caution in forming his opinions upon the subject; he states the facts upon which he reasoned, endeavours to appreciate their value, and after maturely considering the premises, he deduced from them his conclusion. He proceeds upon the principle, that all the secretions are ready formed in the blood, but did not appear to think it necessary to inquire by what means they were origi nally generated there, or how they were introduced into it. Assuming, however, their existence, he conceives that there are seven causes which may contribute to their separation; 1. a difference in the nature of the blood itself; 2. the velocity of the blood as caused by the size of the vessel; 3. the transmission of the blood from one vessel to another which differs from it in size; 4. the angles at which the secreting arteries pass off from the trunk; 5. the course of the vessel, whether straight or winding; 6. the density of the vessel; and lastly, the structure of the excretory duct. There may be some foundation for all these causes; as affecting the contents of the vessels,

One of the rules to be observed is as follows; "You are to dose so much of the medicine as is spent on the stomach and intestines, directly as the constitution; and so much as is carried into the blood, as the square of the constitution, and the sum into the person's size is the quantity required;" p. 35. A less extravagant, but equally unfounded attempt is that of Gorter, in which he refers the whole affair of secretion to the physical properties of the fluid and the size of the vessels; De Secretione; see particularly the accompanying plate.

although we shall probably conceive of them as very inadequate to produce the variety of substances that we meet with. In fact they may be all referred to the size of the vessels, and the velocity with which their contents are propelled through them; the formation of the substances, by the intervention of external agents, or the action of the constituents upon each other having been either not contemplated, or not conceived to form a necessary part of the hypothesis.

As animal chemistry was more attended to, and we became better acquainted with the changes which the component parts of the blood are capable of experiencing, by subjecting them to various chemical re-agents, it was conceived that all the secretions might be produced solely by the operations of chemical affinity. It does not very clearly appear with whom this theory of secretion originated. Perhaps Keill was the first who endeavoured to explain secretion upon chemical principles, but his opinions were altogether so imperfect, as to bear but little resemblance to the modern doctrine."

The main argument for this hypothesis is, that by

7 Haller, El. Phys. vii. 3. 33. Keill, in his treatise on animal secretion, proposes to illustrate the following positions; "I. To show how the secretions are formed in the blood, before they come to the place appointed for secretion; 2. To demonstrate in what manner they are separated from the blood by the glands." p. 1. The 4th of the essays in the "Testamina Medicophysica" is nearly a translation of the above. The reasoning is strictly mathematical, and affords a very remarkable specimen of the misapplied learning of the mathematical physiologists. The doctrine of attraction, as applied to secretion, is particularly aid down in the 7th and 8th prop.

certain chemical processes, we are able to form from the blood, out of the body, substances similar to the secretions; hence, upon the principle of not unnecessarily multiplying causes, it is said that the same kind of operation must produce the secretions in the body. And this, it is supposed, may be effected by conveying the blood in its entire substance, or any of its components separately, as occasion requires, to different parts of the system, there to be subjected to the action of external agents; or by placing the entire blood, or any of its components, in such ́a situation, as that it may undergo the spontaneous changes to which it is liable. An argument has been urged in support of the chemical hypothesis, derived from our knowledge of the variety of substances which may be produced from only a very few elements, merely by their being united in different proportions. This is very remarkably the case with oxygen and nitrogen, which in one proportion compose atmospheric air, in another nitrous oxide, in a third nitric oxide, in a fourth nitrous acid, and in a fifth nitric acid, substances which differ from each other, at least as much as the secretions differ from each other, and from the blood. And what is still more in point, some of the late investigations into the atomic constitution of animal substances, exhibit the same production of new compounds by a different proportion of their elements. Dr. Prout has, with his usual sagacity, developed this kind of relation between the three substances, urea, lithic acid, and sugar, and shown how they may be converted into each