Of to say in his paper on this subject, "there are few dead Digestion. bodies in which the stomach at its great end is not in some degree digested."

165 but the

tion.

That the chyle is not formed there, and that it is chyle form- commonly formed before the food passes through the py ed in the lorus, is proved by the result of some experiments made pyloric por- by Mr Hunter upon dogs, in the year 1760. The dogs were killed while digestion was going on; and in all, the food was least dissolved, or even mixed, towards the great end of the stomach, but became more and more so towards the pylorus, just within which it was mixed with a whitish fluid like cream.

166 Solvent liquor se

From the result of these experiments, as well as from the analogy of other animals, it is reasonable to believe, creted by that the glands situated at the termination of the cuticuglands near lar lining of the oesophagus, which are described by Mr the gullet. Home, secrete the solvent liquor, which is occasionally poured on the food, so as to be intimately mixed with it before it is removed from the cardiac portion; and the muscular contraction retains it there, till this takes place. Such contraction being occasionally required in the stomach, accounts for its being more or less bent upon itself, as by this structure it is more readily divided into two portions, by the action of the muscular fibres at that part where the angle is formed.

167 Curvature

of the sto counted for.

mach ac

168 Ornitho

This contraction also explains why the contents of the stomach are not completely discharged from the first effect of an emetic; and by it Mr Home thinks we may explain the cramp of the stomach, and some kinds of indigestion.

After comparing the stomachs of several carnivorous rinchus the animals with that of man; in tracing the gradation from real, link carnivorous beasts through the bat tribe to birds of prey, between quadrupeds Mr Home remarks, that "the only real link between and birds. the stomachs of quadrupeds and birds is that of the ornithorinchus (or platypus), which, however, is more an approach to the gizzard, being lined with a cuticle containing sand, and having the same relative situation to the oesophagus and duodenum. The food of this animal is not known; it is probably of both kinds; the papilla at the pylorus, which appear to be the secretory ducts of glands, are peculiar to it.

160 General

From the facts and observations brought forward in conclusions. this valuable paper, Mr Home deduces the following general conclusions. "That the solvent liquor is secreted from glands of a somewhat similar structure in all animals, but much larger and more conspicuous in some than in others.

"That these glands are always situated near the orifice of the cavity, the contents of which are exposed to their secretion.

"That the viscid substance found on the internal membrane of all the stomachs that were examined recently after death, is reduced to this state by a secretion from the whole surface of the stomach, which coagulates albumen. This appears to be proved, by every part of the fourth cavity of the calf's stomach having the property of coagulating milk.

"This property in the general secretion of the stomach leads to an opinion, that the coagulation of fluid substances is necessary for their being acted on by the solvent liquor; and a practical observation of the late Mr Hunter, that weak stomachs can digest ouly solid food, is in confirmation of it.

"That in converting animal and vegetable substances

into chyle, the food is first intimately mixed with the Of general secretions of the stomach, and after it has been Digestion acted on by them, the solvent liquor is poured upon it, by which the nutritious part is dissolved. This solution is afterwards conveyed into the pyloric portion, where it is mixed with the secretions peculiar to that cavity, and converted into chyle.

"The great strength of the muscles of the pyloric portion of some stomachs will, by their action, compress the contents, and separate the chyle from the indigestible part of the food.

"In animals whose food is easy of digestion, the sto mach consists of a cardiac and pyloric portion only; but in those whose food is difficult ef digestion, other parts are superadded, in which it undergoes a preparation before it is submitted to that process."

170

The action of the juices of the stomach, or of what Action of we call the gastric juice, appears to have much more ef- the gastric fect in the process of chymification, than the muscular juice, action of the stomach, though the dissolving power of this fluid seems to be proportionally less in those animals that have the most muscular stomachs. The ga stric juice of granivorous birds is capable of dissolving flesh; but when this is entire, it requires four or five days for solution; whereas when bruised, half that time is sufficient. Even grain is not dissolved in it except when bruised. The gastric juice of animals with intermediate stomachs dissolves flesh and cartilage, but not bone. It is incapable of dissolving entire seeds. In animals with membranous stomachs, the gastric juice is extremely active, and seems to be almost the only agent in the digestive process. In some of these animals, however, as the ruminating tribes, this fluid has no effect on the food, unless it be bruised, or thoroughly masticated. Spallanzani found, that owls digest flesh and bones, but not grain ;-that the gastric juice of the eagle dissolves bread and bone, and even animal and vegetable matters, when it is taken out of the body;— that a wood pigeon may be gradually brought to live on flesh;-that the owl and falcon do not digest bread; that the gastric juice of the dog dissolves even the enamel of the teeth.

Hence, in every order of animals, the gastric juice is the principal cause of digestion, and it agrees in all in many properties, and differs in others. In the frog, the newt, scaly fishes, and other cold-blooded animals, it produces digestion in a temperature nearly equal to that of the atmosphere. In warm-blooded animals it is capable of dissolving the aliment in a degree of heat lower than that of these animals. In them too the food is digested in a few hours, whereas in the opposite kind it requires several days, and even weeks, particularly in serpents ; likewise, the gastric juice of the gallinaceous class can dissolve only bodies of a soft and yielding texture, and previously triturated: whilst in others, as serpents, the heron, birds of prey, and the dog, it decomposes substances of great tenacity, as ligaments and tendons; and even of considerable hardness, as the most compact bone. Man belongs to this class, but his gastric juice seems to have no action on the hardest kinds of bones. Some species, likewise, are incapable of digesting vegetables, as birds of prey; but man, the dog, cat, crows, &c. dissolve the individuals of both kingdoms alike, and are omnivorous, and in general their gastric juices produce these effects out of the body.

For

171

Solubility of foods.

The process of chymification depends also, in a great measure, on the nature of the substances employed as food, as some of these are much more soluble than others. On this subject much information may be de*Stark's rived by consulting the experiments of Dr Stark, and those of M. Gosse of Geneva, an abstract of which is given in Johnson's Animal Chemistry, vol. i. p. 207. From the latter experiments it appears, 1st, That the following substances are either insoluble, or are not digest ed in the usual time in the stomach.

Works.

2.

Animal substances. 1. Tendinous parts. 2. Bones. 3. Oily or fatty parts. 4. Indurated white of egg. Vegetable substances. 1. Oily or emulsive seeds. Expressed oils of different nuts and kernels. 3. Dried grapes. 4. Rind of farinaceous substances. 5. Pods of beans and pease. 6. Skins of stone fruits. 7. Husks of fruit, with grains or seeds. 8. Capsules of fruit, with grains. 9. Ligneous stones of fruits. 10. The gastric juice does not destroy the life of some seeds; hence bitter-sweet, hemp, misletoe, and other plants which sometimes grow upon trees, are produced by the means of the excrements of birds, the kernels of seeds being defended from the menstruum by their exterior covering. 2d, That the following are partly soluble, viz. Animal substances. 1. Pork dressed various ways. 2.

Black puddings. 3. Fritters of eggs, fried eggs and

bacon.

Vegetable Substances.-1. Salads of different kinds, rendered more so when dressed. 2. White of cabbage less soluble than red. 3. Beet, cardoons, onions, and leeks. 4. Root of scurvy-grass, red and yellow carrots, succory, are more insoluble in the form of salad than any other way. 5. Pulp of fruit with acids, when not fluid. 6. Warm bread and sweet pastry, from their ducing acidity. 7. Fresh and dry figs. By frying all these substances in butter or oil they become still less soluble. If they are not dissolved in the stomach, they are, however, in the course of their passage through the intestines.

pro

3d, That the following are soluble, or easy of digestion, being generally reduced to chyme in an hour, or an hour and a half.

Animal Substances.-1. Veal, lamb, and in general the flesh of young animals, are sooner dissolved than that of old. 2. Fresh eggs. 3. Cows milk. 4. Perch boiled with a little salt and parsley. When fried or seasoned with oil, wine, and white sauce, it is not so soluble.

Vegetable Substances.1. Herbs, as spinach, mixed with sorrel, are less soluble. Celery. Tops of asparagus, hops, and the ornithogalus of the Pyrenees. 2. Bottom of artichokes. 3. Boiled pulp of fruits, seasoned with sugar. 4. Pulp or meal of farinaceous seeds. 5. Different sorts of wheaten bread, without butter, the second day after baking; the crust more so than the crumb. Salted bread of Geneva more so than that of Paris without salt; brown bread in proportion as it contains more bran is less soluble. 6. Rapes, turnips, potatoes, parsnips, not too old. 7. Gum arabic, but its acid is soon felt. The Arabians use it as food.

The solvent power of the gastric juice is increased by various stimulants, especially by those called condiments, as sea salt, spices, mustard, vinegar, as well as by viVOL. XVI. Part II.

172

nous and spirituous liquors and old cheese in small quan- Of tities, and by various bitters. It is retarded by large Digestion. quantities of diluting liquors, especially when taken hot by acids and astringents taken a short time after eating; by unctuous substances; by mental employment, or severe bodily exercise, too soon after a meal; and by leaning with the breast against a table. It may be proper here to notice the various opinions Various that have been entertained respecting the immediate digestion. cause of digestion. The principal of these opinions are, that it is produced by coction or heat; by trituration in the stomach; by fermentation, or by putrefaction. 173 That it is not brought about by heat alone, will appear from the circumstance, that many cold-blooded animals digest their food as completely, though not so expeditiously, as warm-blooded animals.

theories of

Heat.

174

tion.

177

On the whole it appears, that in most animals the digestion of food in the stomach depends partly on a due degree of heat, partly on the vital action of the stomach, but chiefly on the action of the gastric juice. When the aliments have been converted in the sto- Chylifica mach to the crude pulp called chyme, they are gradually propelled through the pylorus into the duodenum, where they are mixed with the bile, the pancreatic juice, and the fluids that are separated by the mucous coat of that intestine, and are thus reduced to a still finer pulp, containing, as one of its principal ingredients, the nutritious fluid called chyle, the nature and properties of which, as they have been but slightly mentioned in the former parts of this work, fall to be noticed here.

178

The properties of chyle have not been minutely in- Properties of chyle. vestigated; but according to Fordyce, as far as experiment has been carried, the chyle of quadrupeds is so similar to that of man, and of each other, as hardly to be distinguished, even in tribes the most opposite to each other in their structure, food, and habits of life. As far as we can perceive, the chyle of a dog or a wolf differs in nothing from that of a sheep or an ox.

The chyle consists of three parts; one part which is fluid, and contained in the lacteals, but coagulates on extravasation.

The second part consists of a fluid, which is coagulable by heat, and in all its properties hitherto observed, it is similar to the serum of the blood.

The third part consists of globules, which render the whole white and opake. These globules have been supposed by many to be an expressed oil; but this has not been proved. Neither has it been perfectly demonstrated that sugar is contained in the chyle, although it has + 3 Q

been

Of been made very probable. The difficulty of determinDigestion ing these points arises from the small quantity that can be collected, the largest animals not supplying more than one ounce or two, at the most. However, the part coagulating on extravasation; the part agreeing with seFordyce rum in its qualities; the globular part, which in some on Dige animals, but not in quadrupeds, exists without giving whiteness to the chyle-alone, or along with sugar, form the essential parts of the chyle *.

stion.

179 Progress of the chyle.

180 Passage of food

intestines,

The compound pulpy matter containing the chyle is carried forward from the duodenum through the whole course of the intestines, where it is subjected to the continual action of the internal wrinkled membrane of the bowels, and its nutritious particles, or chyle, selected and absorbed by the lacteals that are abundantly distributed there, and open their mouths directly within the cavity. As to the movements of the alimentary canal, the through the direction of hairs found in the stomach, and the balls of hairs which are thrown up, would appear to indicate a circular motion. The intestinal part has a motion similar to that of a worm, and is called the vermicular or peristaltic. Here every portion retains its own motion, although it be separated from the rest by ligatures. The stomach of the polype, the gullets of the ruminating kinds, and the cæca, have the motion in different directions at different times; and that observed in the alimentary canal of a louse is, when viewed through a microscope in the time of action, amazingly rapid; the stimulating causes employed are the food, the different liquors with which it is mixed, the air, the nerves where they exist, and a portion of heat. Some degree of heat is necessary to every process of digestion, both in the animal and vegetable kingdom; what that degree is depends on the nature of the living body; and is various according to its age, its health, its employments and habits.

191

Relations

With respect to the function of digestion in the lower classes of animals, we can say but little. We know that their food is dissolved in the stomachs of the crustacea, of mollusca, and of polypes; but whether this process in most insects and worms is any thing more than imbibition, or taking in aliment, which is to undergo little change, we are uncertain. We know, indeed, that We know, indeed, that many insects live on substances which must be dissolved before they enter into the pores of their bodies, and that many of them abound in acrid juices, which are well fitted for this solution. It does not appear that plants possess what may be called the faculty of digestion.

The relations between digestion and the functions we between di- have already considered, especially sensation, are various gestion and and important. The sympathies that exist between the

sensation.

head and the stomach, have been long acknowledged. Several affections of the brain are accompanied with sickness at the stomach, loss of appetite, and indigestion; while on the other hand, the deranged state of the digestive organs seldom fails to produce giddiness, headach,

of

ringing in the ears, confusion or depravation of sight, &c.; and if the former symptoms arise to a great height, Digestion as in the case of overloaded stomach or surfeit, coma, or even apoplexy, is frequently produced. In many nervous affections, particularly hysteria and hypochondriasis, in which there frequently takes place astonishing accumulations of air in the stomach and bowels, the affections of the head, such as stupor, confusion of thought, partial blindness, &c. sometimes proceed to such a height, as to threaten, or even sometimes to produce, an apoplectic paroxysm. In many cases these affections are referred immediately to the head; but are proved, in most instances, to depend on the disordered state of the alimentary canal, from the immediate relief procured by those remedies which promote the discharge of air, or produce copious evacuation from the bowels. On the other hand, in some diseases, where the head is primarily affected, as in phrenitis hydrocephalica (water in the head), the complaint is referred to the bowels, from the costiveness or other disordered state of these. The daily experience of literary men shows how much intense thought diminishes the digestive powers, and how imperfectly studious occupations can be carried on after a full meal. The action of the digestive organs is also considerably influenced by the mind, or the passions. We know how readily the appetite may be diminished or destroyed by sudden anger or affliction.

The action of the stomach may even be influenced by the will. We have known a person who could vomit whenever he pleased; and Dr Darwin speaks of another who had acquired this voluntary command over the inverted motions of the stomach and throat, to such a degree, as to gain a subsistence by exhibiting these unnatural powers to the public. At these exhibitions he was accustomed to swallow a pint of red rough gooseberries, and a pint of white smooth ones; to bring them up in small parcels into his mouth, and restore them separately to the spectators, who called for red or white, as they Zoonopleased, till the whole were redelivered*.

mia, vol i.

Plate

The sympathies that take place between the brain and p. 276. the digestive organs, are easily explained, from considering the distribution of the great sympathetic nerve, to illustrate which we have given a figure (fig. 1.) show-ccccvL ing its course and distribution from the head through the chest, as far as the stomach.

182

The relations between the digestive and the locomo- Relations tive functions, are not less obvious. Experience shews between how much digestion depends on regular exercise, and digestion how imperfectly it is carried on in the stomach of the and motion. indolent and sedentary; while on the other hand, when the stomach is overloaded, voluntary motion becomes difficult and fatiguing. Spasmodic contractions of the muscles, twitchings of the limbs, and similar affections, are the common attendants of indigestion, though these may perhaps be referred equally to the nervous as to the muscular system.

183

The principal morbid affections of digestion are, nau-Morbid st sea, flatulence, eructation, rumination (G), vomiting, fections of heartburn, digestion.

man.

Of was not observed to have lost any thing during immerAbsorption. sion in the warm bath.

* Medical

Report, first edit. p. 227.

Now, during these experiments, the body was doubt less wasting, by the pulmonary and cutaneous discharges, and yet the weight of the body either continued unchanged; or where a loss of weight was observed, this was constantly less, greatly less, than is experienced during the same interval in air. And we might be inclined to infer, from a truth so general, and so well ascertained, an argument in favour of absorption.

It might be argued, that the loss of weight amounts to little or nothing, because, during immersion, the body acquires more by inhalation than it does or can do in the air; that the loss by the pulmonary and cutaneous discharges are counterbalanced, or nearly counterbalanced, by the increased absorption.

Those, however, who deny absorption, will not allow us the advantage of this argument. They tell us, that the exhalation by the skin and lungs is diminished, which sufficiently explains why the body loses less in the warm bath than in air. But that the accustomed discharges are suppressed or diminished in the warm or tepid bath, is, we apprehend, far from being proved; and, till this proposition is made good, the argument against cutaneous inhalation cannot be securely maintained.

One of Dr Currie's cases deserves farther consideration. We allude to the case of dysphagia, published by this gentleman*, in which Mr M. the subject of the case, was several times immersed in a warm bath of milk and water, and was weighed when taken out. Mr M. it is true, gained no weight while in the warm bath but the loss continually going on in the air was, as in other trials, suspended during the immersions. Besides, he also expressed great comfort from the bath, with abatement of thirst; and, subsequent to the daily use of it, the urine flowed more plentifully, and became less pungent. An observation, precisely similar, is made by Mr Cruickshank. "A patient of mine (says Mr Cruickshank), with a stricture of the oesophagus, received nothing, either solid or liquid, into the stomach for two months; he was exceedingly thirsty, and complained of making no water. I ordered him the warm bath for an hour, evening and morning, for a month; his thirst vanished, and he made water in the same man+ Cruick- ner as when he used to drink by the mouth. (H)†.

shank on

the Absor bents.

But to return to the case of Mr M. Dr Currie himself remarks, that the discharge by urine alone exceeded much in weight the waste of his whole body; and it cannot be doubted that the discharge by stool and perspiration exceeded the weight of the clysters. Thus it appears, that the egesta exceeded the ingesta in a proportion much greater than the waste of his body will explain. How is this accounted for, Dr Currie asks, unless by cutaneous absorption?

That the excess of these discharges above the ingesta and total waste, can be accounted for by absorption only, was indeed an irresistible conclusion. Still, how

Of

ever, cutaneous absorption is denied ; and, when forced to confess that there are cases where the egesta exceed Absorption. the ingesta in a much greater proportion than the waste of the body will explain, and which can only be accounted for by absorption, they refuse this function to the skin, and bestow it most liberally, and, in so far as we know, most gratuitously, on the lungs. We are not entitled, in return, to deny the reality of pulmonary absorption, but we may surely be allowed to urge, that there is no proof that the only inhaling organ is in the lungs; and there is none against the possibility of cutaneous absorption.

Is it not, on the other hand, proved, by the experiments of Seguin and Lavoisier, that the exhalation greatly exceeds the absorption by the pulmonary system? And if this is always the case, we cannot explain by pulmonary inhalation alone, why the egesta should, in some cases, exceed the ingesta in a much greater proportion than the waste of body will account for.

We now proceed to examine another class of experiments, much insisted on by those who deny cutaneous absorption; we mean those experiments performed by immersing a part of the body in solutions of active drugs, the absorption of which should be indicated by their usual effect on the system.

Seguin made numerous experiments of this kind with solutions of muriate of mercury (corrosive sublimate), on syphilitic patients. And we are informed, that in cases where the epidermis was perfectly sound, neither the known effects of mercury on the body, nor any amelioration of the venereal symptoms, was ever observed.

He also immersed his own arm in a solution of two drams of the mercurial muriate in ten pounds of water. At the temperature of 10° and 28° Reaumur, no part of the salt was missing at the end of the experiment; but when the bath was at 18° of the same scale there was a loss of one or two grains of the muriate in the bour, though the quantity of fluid was not diminished.

The explanation given by Seguin of this unexpected result is curiously ingenious, but embarrassed, and inconsistent.

At the temperature of 12°, he observes, the exhalants are in a state of contraction, and their orifices nearly closed. When the heat again is raised to 26°, the exhalation is so rapid, that nothing can enter the vessels from without; but at 18° of temperature, the orifices of the exhalants are sufficiently relaxed, and the exhalation at the same time so conveniently languid, that the solution rests quietly in contact with the matter of perspiration in the mouths of the exhalants, where it is somehow or other decomposed; a part of the salt leaving the water of solution, and combining with the perspirative matter, with which it is carried into the system. Carried into # La Med. the circulation by the exhalants! Is not this a plain ac- Eclairée, knowledgment of the reality of inhalation? But if in tom. iii. one case substances may thus be carried into the circu-P. 238, lation, why not in many others?

Surely if the weight continues undiminished, in cir

cumstances