presence； the body of a patient of his own； where the stomach was in part dissolved； which appeared to him very unaccountable； as there had been no previous symptom that could have led him to suspect any disease in the stomach。 I took that opportunity of giving him my ideas respecting it； and told him that I had long been making experiments on digestion； and considered this as one of the facts which proved a converting power in the gastric juice。 。 。 。 There are a great many powers in nature which the living principle does not enable the animal matter； with which it is combined； to resistviz。； the mechanical and most of the strongest chemical solvents。 It renders it； however； capable of resisting the powers of fermentation； digestion； and perhaps several others； which are well known to act on the same matter when deprived of the living principle and entirely to decompose it。 〃
Hunter concludes his paper with the following paragraph： 〃These appearances throw considerable light on the principle of digestion； and show that it is neither a mechanical power； nor contractions of the stomach； nor heat； but something secreted in the coats of the stomach； and thrown into its cavity； which there animalizes the food or assimilates it to the nature of the blood。 The power of this juice is confined or limited to certain substances； especially of the vegetable and animal kingdoms； and although this menstruum is capable of acting independently of the stomach； yet it is indebted to that viscus for its continuance。＇5＇
THE FUNCTION OF RESPIRATION
It is a curious commentary on the crude notions of mechanics of previous generations that it should have been necessary to prove by experiment that the thin； almost membranous stomach of a mammal has not the power to pulverize； by mere attrition； the foods that are taken into it。 However； the proof was now for the first time forthcoming； and the question of the general character of the function of digestion was forever set at rest。 Almost simultaneously with this great advance； corresponding progress was made in an allied field： the mysteries of respiration were at last cleared up； thanks to the new knowledge of chemistry。 The solution of the problem followed almost as a matter of course upon the advances of that science in the latter part of the century。 Hitherto no one since Mayow； of the previous century； whose flash of insight had been strangely overlooked and forgotten； had even vaguely surmised the true function of the lungs。 The great Boerhaave had supposed that respiration is chiefly important as an aid to the circulation of the blood； his great pupil； Haller； had believed to the day of his death in 1777 that the main purpose of the function is to form the voice。 No genius could hope to fathom the mystery of the lungs so long as air was supposed to be a simple element； serving a mere mechanical purpose in the economy of the earth。
But the discovery of oxygen gave the clew； and very soon all the chemists were testing the air that came from the lungsDr。 Priestley； as usual； being in the van。 His initial experiments were made in 1777； and from the outset the problem was as good as solved。 Other experimenters confirmed his results in all their essentialsnotably Scheele and Lavoisier and Spallanzani and Davy。 It was clearly established that there is chemical action in the contact of the air with the tissue of the lungs； that some of the oxygen of the air disappears； and that carbonic…acid gas is added to the inspired air。 It was shown； too； that the blood； having come in contact with the air； is changed from black to red in color。 These essentials were not in dispute from the first。 But as to just what chemical changes caused these results was the subject of controversy。 Whether； for example； oxygen is actually absorbed into the blood； or whether it merely unites with carbon given off from the blood； was long in dispute。
Each of the main disputants was biased by his own particular views as to the moot points of chemistry。 Lavoisier； for example； believed oxygen gas to be composed of a metal oxygen combined with the alleged element heat； Dr。 Priestley thought it a compound of positive electricity and phlogiston； and Humphry Davy； when he entered the lists a little later； supposed it to be a compound of oxygen and light。 Such mistaken notions naturally complicated matters and delayed a complete understanding of the chemical processes of respiration。 It was some time； too； before the idea gained acceptance that the most important chemical changes do not occur in the lungs themselves； but in the ultimate tissues。 Indeed； the matter was not clearly settled at the close of the century。 Nevertheless； the problem of respiration had been solved in its essentials。 Moreover； the vastly important fact had been established that a process essentially identical with respiration is necessary to the existence not only of all creatures supplied with lungs； but to fishes； insects； and even vegetablesin short； to every kind of living organism。
ERASMUS DARWIN AND VEGETABLE PHYSIOLOGY
Some interesting experiments regarding vegetable respiration were made just at the close of the century by Erasmus Darwin； and recorded in his Botanic Garden as a foot…note to the verse：
〃While spread in air the leaves respiring play。〃
These notes are worth quoting at some length； as they give a clear idea of the physiological doctrines of the time （1799）； while taking advance ground as to the specific matter in question：
〃There have been various opinions；〃 Darwin says； 〃concerning the use of the leaves of plants in the vegetable economy。 Some have contended that they are perspiratory organs。 This does not seem probable from an experiment of Dr。 Hales； Vegetable Statics； p。 30。 He； found； by cutting off branches of trees with apples on them and taking off the leaves； that an apple exhaled about as much as two leaves the surfaces of which were nearly equal to the apple； whence it would appear that apples have as good a claim to be termed perspiratory organs as leaves。 Others have believed them excretory organs of excrementitious juices； but as the vapor exhaled from vegetables has no taste； this idea is no more probable than the other； add to this that in most weathers they do not appear to perspire or exhale at all。
〃The internal surface of the lungs or air…vessels in men is said to be equal to the external surface of the whole body； or almost fifteen square feet； on this surface the blood is exposed to the influence of the respired air through the medium； however； of a thin pellicle； by this exposure to the air it has its color changed from deep red to bright scarlet； and acquires something so necessary to the existence of life that we can live scarcely a minute without this wonderful process。
〃The analogy between the leaves of plants and the lungs or gills of animals seems to embrace so many circumstances that we can scarcely withhold our consent to their performing similar offices。
〃1。 The great surface of leaves compared to that of the trunk and branches of trees is such that it would seem to be an organ well adapted for the purpose of exposing the vegetable juices to the influence of the air； this； however； we shall see afterwards is probably performed only by their upper surfaces； yet even in this case the surface of the leaves in general bear a greater proportion to the surface of the tree than the lungs of animals to their external surfaces。
〃2。 In the lung of animals the blood； after having been exposed to the air in the extremities of the pulmonary artery； is changed in color from deep red to bright scarlet； and certainly in some of its essential properties it is then collected by the pulmonary vein and returned to the heart。 To show a similarity of circumstances in the leaves of plants； the following experiment was made； June 24； 1781。 A stalk with leaves and seed…vessels of large spurge （Euphorbia helioscopia） had been several days placed in a decoction of madder （Rubia tinctorum） so that the lower part of the stem and two of the undermost leaves were immersed in it。 After having washed the immersed leaves in clear water I could readily discover the color of the madder passing along the middle rib of each leaf。 The red artery was beautifully visible on the under and on the upper surface of the leaf； but on the upper side many red branches were seen going from it to the extremities of the leaf； which on the other side were not visible except by looking through it against the light。 On this under side a system of branching vessels carrying a pale milky fluid were seen coming from the extremities of the leaf； and covering the whole under side of it； and joining two large veins； one on each side of the red artery in the middle rib of the leaf； and along with it descending to the foot…stalk or petiole。 On slitting one of these leaves with scissors； and having a magnifying…glass ready； the milky blood was seen oozing out of the returning veins on each side of the red artery in the middle rib； but none of the red fluid from the artery。
〃All these appearances were more easily seen in a leaf of Picris treated in the same manner； for in this milky plant the stems and middle rib of the leaves are sometimes naturally colored reddish； and hence the color of the madder seemed to pass farther into the ramifications of their leaf…arteries； and was there beautifully visible with the returning branches of milky veins on each side。〃
Darwin now goes on to draw an incorrect inference from his observations：
〃3。 From these experiments；〃 he says； 〃the upper surface of the leaf appeared to be the immediate organ of respiration； because the colored fluid was carried to the extremities of the leaf by vessels most conspicuous on the upper surface； and there changed into a milky fluid； which is the blood of the plant； and then returned by concomitant veins on the under surface； which were seen to ooze when divided with scissors； and which； in Picris； particularly； render the under surface of the leaves greatly whiter than the upper one。〃
But in point of fact； as studies of a later generation were to show； it is the under surface of the leaf that is most abundantly provided with stomata