decomposed, so as to acquire new sensible properties which they impart to the solids or fluids of the ani mal. Turpentine presents us with a remarkable example of this kind, which, when taken into the stomach, imparts to the urine an odour exactly resembling that of violets."

In the last chapter I offered some remarks upon the various salts which are found in the blood, and I discussed the question whether we are able to account for the quantity which exists in the different solids and fluids, by supposing them to be introduced along with the food. The facts of which we are in possession, would lead us to the conclusion, that the quantity of these earths or salts that are introduced into the stomach ab extra, is not sufficient to supply the demands of the system, but that they must, in some way or other, be generated by the vital powers. Our next inquiry will be, by what powers, or in what part of the system, are they produced; they may either be formed by the digestive organs, during the process of chymification or chylification, or by a process more analogous to secretion, by an organ or organs expressly appropriated to the purpose. The difficulties which

3 Fordyce states that indigo and musk are both taken up by the lacteals, so far retaining their previous properties, as that the former possesses its specific colour, and the latter its peculiar odour; On Digest. p. 122. This power is, however, limited to certain substances, while others, although equally exposed to the action of the lacteals, are incapable of entering them, these vessels thus exercising what appears to be a kind of selection; p. 123; but upon this subject I shall have occasion to offer some further observation in the next chapter. ·

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exist upon both these suppositions are very great; we can indeed form no conception of the nature of the operation in either case, and it is only in consequence of the necessity that there is of attempting some explanation, that we have recourse to them. The subject is again introduced into this place for the pose of inquiring whether there be any grounds of preference between these two opinions, and we may go so far as to remark, that if we find in the chyle all the salts that exist, in any of the constituents of the body, we must conclude that they are produced in the process of digestion, and afterwards merely separated from the blood; a conclusion to which the results of our experiments would seem to lead us. The whole subject is, however, one that is so extremely obscure, and of such difficult solution, that I do not think it desirable to enter more particularly into the consideration of it, until we are in possession of a more firm basis of facts, on which to build our hypotheses.

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§ 4. Theory of Digestion.

There are few subjects in physiology that have afforded a more fruitful subject for speculation than the theory of digestion, for in this, as in other parts of the science, we may remark, that the more obscure is the subject, and the less real information we possess concerning it, the more numerous have been the attempts to frame hypotheses to account for it. The opinion proposed by Hippocrates, and adopted by Galen and the ancients generally was, that the aliment is digested by what is termed concoction. But

this is to be considered rather as another word ex pressive of the action, than as any explanation of it. It is, indeed, synonymous with the term digestion, and derived from the same analogy of the change which substances undergo, when they are exposed to a certain degree of temperature in close vessels.*

The next hypothesis was that of putrefaction, an hypothesis which was maintained by some of the earlier chemists, and was supported by various obsér. vations, and even experiments, that were supposed to be favourable to it. The food, when it is received into the stomach, was observed to have its texture broken down, and to have acquired an unpleasant odour, which the older physiologists, according to the loose mode of reasoning which they employed, regarded as a species of putrefaction. It is a sufficient refutation of this hypothesis to remark, that digestion and putrefaction are processes of a totally different nature;

4 See Boerhaave, Prælect. not. ad. § 86. t. i. p. 158, 160; Blumenbach, Inst. Phys. § 360. p. 202. By the following passage in Celsus, it appears that the hypothesis of attrition and of putrefaction, had also their defenders among the ancients; "Duce, alii, Erasistrato, atteri cibum in ventre contendunt: alii, Plistonico Praxagoræ discipulo, putrescere: alii credunt Hippocrati, per calorem cibus concoqui." Præf. p. 6. § 10. The student, who is disposed to make himself acquainted with the doctrines of the earlier physiologists on the subject of concoction, à process which was supposed to be concerned in other functions besides that of digestion, may consult the treatise of Fernel, Physiol. lib. 6. c. 6. "de concoctionibus." Fernel was one of the first of the moderns who wrote from his own observations, and who exercised his own judgment on subjects of physiology and pathology.

and that so far from their having any connexion with each other, one of the first effects of the gastric juice is to resist putrefaction, or even to suspend it, if it has actually commenced."

The mechanical physiologists endeavoured to account for all the phenomena of digestion by trituration, and they performed many curious experiments in proof of their opinion on those animals which are said to possess muscular stomachs. But although the facts were correctly stated, the conclusions which were deduced from them were inaccurate in two respects. In the first place, they extended to all classes of animals an action which belongs to certain species only, and secondly, in considering the trituration of the muscular stomach as analogous to the process of chymification in membranous stomachs. I have already stated, that the operation of the gizzard is entirely mechanical, and is equivalent to the teeth of

This hypothesis has had its advocates even in modern times; Cheselden, Anat. p. 155,' says " digestion is no other than corruption or putrefaction of our food." It would appear to have been invented by Plistonicus, of whom nothing more is known than that he was the author of this hypothesis; see Celsus, ut supra, and Le Clerc, Hist. de la Med. part. 2. liv. 1. c. 8. p. 326, 7. The hypotheses of Pringle and M'Bride, although nominally founded upon fermentation, ought really to be referred to putrefaction. M'Bride and most of his contemporaries thought that the saliva was an active promoter of this decomposition, Essays, p. 16, 7: whereas Pringle's experiments; Appendix, p. 362; led him to conclude that saliva resists this process. With respect to the gastric juice, the fact appears to be that it is decidedly antiseptic; Stevens, c. 9; Spallanzani, Exper. § 249,,259.

quadrupeds; the food is then brought into the same state as it is by mastication, and has still to undergo the action of the proper digesting stomach. If direct facts were wanting to confirm this opinion, they are abundantly furnished by the experiments of Stevens and Spallanzani, as they prove that chymification is effected under circumstances in which trituration could not possibly operate."

In opposition to the mechanical doctrine of trituration, an opinion was advanced by the earlier chemists, that the action of the stomach consisted in a species of fermentation. This hypothesis appears to

6 Physiological speculation was, perhaps, never carried to a greater excess than by Pitcairn, in the estimate which he makes of the mechanical force which the stomach exercises in digestion. After employing much learned and abstruse discussion to prove that no other power is competent to produce the requisite effect upon the aliment, he calculates that the power of the muscular fibres of the stomach is equal to 12,951 lbs.; Dissert. p. 72..95; also Elem. c. 5. p. 25..7; see the observations of Cheselden, Anat. p. 152..5; also of Hales, who estimates "that 20 lbs. would come nearer to the pressure of the aliments of a full stomach;" Statical Essays, v. ii. p. 174, 5.

Haller very explicitly states the impossibility of trituration being effected by a membranous stomach; El. Phys. xix. 5. 1; yet he scarcely draws a sufficiently accurate line of distinction between the mechanical and chemical action of this organ. It is amusing to observe the learned and laboured arguments which Fordyce thought it necessary to adduce in order to prove that minute mechanical division alone cannot alter the chemical nature of a substance; On Digest. p. 124.. 138. See remarks on trituration by Stevens, De Alim. Concoct. cap. 10; also by Richerand, El. Phys. § 18. p. 100..2.