QUANTITY OF FOOD REQUIRED.

39

of different ele

From the experiments of Boussingault with reference to fat, and of Bidder and Schmidt with reference to the albuminates, and Maximum limof Von Becker with reference to the carbohydrates, we learn it of absorption that only definite quantities of these substances can be ab- ments of food. sorbed by the intestine in definite periods of time. This maximum limit is, however, far more than the necessities of the system require; hence in overfeeding, though much of the excess of food passes away with the excrement, a very large portion is, as it were, needlessly absorbed, and, undergoing metamorphosis in the blood, is removed by the kidneys. To this portion Lehmann applies the designation introduced by Schmidt, luxus consumption, or superfluous consumption. Of course, the simplest condition under which we can investigate the normal quantity of food required is that of an invariable weight, and the difficulties of the inquiry are increased when growth, corpulence, pregnancy, or other such states, are included.

Though we are very far from being able to offer a complete solution of the problem of the amount of food required, in its most general sense, yet, through the labors of many chemists, we have accumulated several facts which have a bearing on this question. Thus it is known that albuminous substances alone can not be absorbed in quantity enough to compensate for the loss of carbon by respiration. A duck, as is shown by Boussingault, expires in one hour 1.25 grammes of carbon, but can only absorb of carbon in albuminates 1.00 gramme. So, in like manner, fat alone is inadequate, for of this substance 0.84 gramme, containing about 0.70 gramme of carbon, can only be taken up in an hour, and this is not much more than half of what the respiratory operation demands. The carbohydrates, however, can be absorbed in sufficient proportion, and in this mixed manner are all the requirements satisfied. Boussingault makes the curious remark that, in the quantity of starch, 5.26 parts, and the quantity of sugar, 5.62 parts, which this bird can absorb in one hour, there are nearly the same quantities, 2.37, of carbon.

Amount of

amount of loss.

Among the special investigations which have been made to determine the amount of food used and the amount of educts from the system, should be mentioned that of Valentin upon himself. food, and His weight was 117 lbs.; his diurnal consumption of food, 6.451 lbs.; solid excrement, .42 lb.; urine, 4.686 lbs.; and 2.751 lbs. perspiration. From the more recent and very exact experiments of Barral, it is inferred that of 100 grammes of carbon which have been absorbed into the organism, 91.59 escape as carbonic acid through the lungs and skin, 4.58 appear in the urine, and 3.83 are re-excreted and appear in the fæces. Upon similar principles, Lehmann computes, from the data furnished by Barral, that for every 100 parts of absorbed nitrogen, 49.6 parts are removed through the skin and lungs, 42.07 are found in

the urine, and 8.33 are re-excreted into the fæces. As a general result, \ it follows, from these experiments, that an adult man oxidizes, on an average, 289 grammes of carbon, and 18.6 grammes of hydrogen in twenty-four hours.

CHAPTER III.

OF DIGESTION.

TISSUE-MAKING OR HISTOGENETIC DIGESTION.

Nature of Digestion.—The Mouth, Teeth, Stomach.—The Salivary Glands.—Different Kinds of Saliva.-Properties of mixed Saliva: its Quantity, Composition, and Functions.-Relation of the Salivary Glands and Kidneys.-The digestive Tract.-The Stomach.-Gastric Juice.Organs for its Preparation.—Manner of producing Chyme.—Influence of the Nerves.-Artificial Digestion.-Preparation and Properties of Pepsin.—Regional and functional Divisions of the Stomach in Animals and in Man.-Object of Stomach Digestion.-Peptones.-Use of Salt. -Digestibility of various Articles of Food.

BEFORE the food can be absorbed and carried to all parts of the sysNature of tem it must be submitted to certain preparatory operations. digestion. Since it is either to be dissolved in the blood or transported as chyle through the lacteal vessels, it is absolutely necessary to bring it into a condition of solution in water, or at least into a state of minute suspension in that liquid. Received in masses of a certain size, it is first cut and crushed into smaller portions by the teeth, and then brought from an insoluble into a soluble or suspended state by the chemical action of the digestive juices.

In the mouth the food is submitted to a twofold preparation. It is Functions of divided by the mechanical action of the teeth, and also simulthe mouth. taneously mingled with liquids secreted from the salivary glands.

The animal series present us with numberless contrivances for accomplishing this comminution. The teeth, though of a bony nature, are not to be regarded as appertaining to the skeleton, but rather to the digestive mechanism. Their structure, number, and position differ very much in different tribes. In certain fishes the mouth is almost lined with them. In crabs they extend to the stomach, but in other cases they are restricted to the pharynx, or are wholly absent; this being the case, for instance, among the ant-eaters. Those insects whose food is of a fluid nature have Instruments of no need of teeth; but those which use solid material are accomminution commodated with suitable instruments of abrasion, such as borers, chisels, saws, nippers, the particular mechanism re

in various ani

mals.

sorted to being adapted to the nature of the food. It is to be understood that these mechanical terms are not mere metaphors, they indicate the actual nature of the apparatus. The object aimed at is to obtain the food in such small portions, and in such a bruised or pulpy condition, that digestion can be accomplished promptly. In man the number of The teeth. temporary teeth is twenty, ten in each jaw. They are arranged

in three classes-four incisors, two canines, and four molars for the upper and under jaw respectively. The permanent teeth, which are eventually substituted for these temporary ones, are thirty-two in number, class

Fig. 1.

ified for each jaw as four incisors, two canines, four bicuspids, and six molars. Their arrangement is exemplified in Fig. 1, representing the lower jaw, in which i is the middle and lateral incisor, c the canine, b the two bicuspids, and m the three molars.

The movements of the teeth, aided by those of the tongue, accomplish a duc abrasion of the food, and simultaneously incorporate it with the saliva. This is, therefore, a purely mechanical operation. It is analogous to Mechanical nathe methods to which chemists resort in their laboratories ture of masticawhen they prepare solid materials for exposure to reagents.

tion.

The mingling of food with saliva, or insalivation, effects a double object. Coated over with a glairy juice, the bruised substance passes along the oesophageal tube into the stomach; but there are also certain chemical changes, which, commencing in the mouth, are of essential importance to the completion of digestion.

stomach.

The stomach is an expansion of the alimentary canal between the œsophagus and duodenum, of a conical figure, the base of Description of which is to the left. It communicates with the oesophagus the human by its cardiac orifice, and by its pyloric with the duodenum. It consists of three coats or tunics-the serous or peritoneal, which is exterior; the muscular, which is intermediate; and the mucous, which is interior. They are connected with each other by cellular tissue. The fibres of the muscular coat run in three different directions, constituting three layers; the superficial ones are longitudinal, radiating from the oesophagus over the surface of the organ; those of the middle layer are circular, or ring-like; they are well developed about the middle of the stomach, and by their contractions sometimes make it assume a divided appearance, as though composed of two compartments. Toward the pylorus they are also greatly re-enforced. The fibres of the third layer take, for the most part, an oblique direction. The interior or mucous coat is some

THE STOMACH.

42 times termed the villous, from its velvety appearance. Its color is very variable; it is folded into rugæ, which admit of variations in the distention of the stomach, without interference with the structure or functions of the membranes of which they are a part. The cardiac orifice is plicated, and the opening into the duodenum is through a circular fold with a central aperture-the pyloric valve, which being surrounded with a band of muscular fibres, acting as a sphincter, the passage from the stomach to the intestine may be entirely obstructed.

Fig. 2.

The stomach is seen in section Fig. 2, a being the œsophagus; b, the greater

extremity; c, the smaller curvature; d, the great curvature; e, the pyloric or less end; f, h, the duodenum; g, place of entry of the ductus communis choledochus and pancreatic duct. The place of junction of the œsophagus is the cardiac region: the

membrane is there plicated. The place of junction of the duodenum is the pyloric region.

In a

The typical form of the digestive apparatus is a sac with one aperture, Types of the which serves the double purpose of affording an entrance to stomach. nutritive material, and an outlet to undigested remains. higher condition it may be conceived of as a tube open at both ends, and having a sac-like swelling on its middle part. The portion of the tube anterior to the sac is the type of the oesophagus, its aperture answering to the mouth, the sac-like swelling being the type of the stomach, and the tube leading from it representing the intestinal canal. In the more elementary of such forms, vessels arise from the walls of the digestive cavity, and pass to all other parts of the system. These serve to convey the elaborated material. Certain appendages are soon to be discovered in connection with this simple digestive mechanism. They are for the preparation of salivary, gastric, pancreatic, or biliary juices. In size or development they vary with the habits of life of the animal, or with the nature of its food. Indeed, the same remark may be made as respects the entire digestive tract of the highest tribes. Thus, in the bat the length of the intestine is to that of the body as three to one, but in the sheep as twenty-eight to one. The ruminants generally have an intesIn man and in monkeys the proportion is Again, as regards construction, there are many

tinal tube of great length. - about five or six to one.

DIFFERENT KINDS OF SALIVA.

43

diversities, the number of digestive dilatations and their size corresponding in some measure to the nature of the food.

Three pairs of glands, the parotid, submaxillary, and sublingual, secrete saliva. Of these organs the parotid is the largest; its Different kinds secretion is delivered through the duct of Steno. The sub- of saliva. maxillary duct is Wharton's, but the sublingual pours its fluid through many small apertures near the frenum linguæ. Besides these proper salivas, the lining membrane of the mouth yields a fluid, the buccal mucus. The parotid saliva is thin and watery, limpid and colorless, inodorous and tasteless. Secreted during fasting or under the use of The parotid sastimulating food, it is denser. It contains so large a quanti- liva.

ty of lime that, on exposure to the air, it becomes covered with an incrustation of the carbonate of that substance. It also contains sulphocyanide of potassium. Its organic ingredient, if not albuminate of soda, closely resembles that body.

From the chemical constitution of the saliva of the parotids, the physiological function of those glands, as aquiparous organs, is established. They yield a certain quantity of watery juice, which, by reason of its thinness or fluidity, is readily incorporated with the food by the teeth. Parotid saliva appears to have no power of transmuting starch into sugar. The submaxillary saliva is also colorless and limpid, tasteless and inodorous. It contains no morphological elements. It is The submaxillighter than the parotid, less alkaline, and contains less lime. lary saliva. For this reason, when exposed to the air, it does not become incrusted with carbonate of that earth. It contains sulphocyanide of potassium. It is so viscid and glutinous that it may be drawn into threads. From this physical property it probably facilitates deglutition by furnishing a kind of anti-friction coating.

The sublingual saliva is thin and watery, containing, like the parotid, but a small percentage of solid matter, and probably dis- The sublingual charging a similar function.

saliva.

Besides the special salivary juices, the lining membrane of the mouth pours forth a liquid-the buccal mucus- -a thick and tena- The buccal mucious substance, having many epithelial cells. It is alkaline cus.

in its reaction, does not coagulate on heating, its insoluble salts containing no carbonate of lime. It has been obtained for examination by tying the ducts of Steno and Wharton, keeping the nostrils open and the head inclined, so that, the animal being unable to swallow, the mucus flows out of the mouth.

The buccal mucus, if mixed with parotid saliva, does not appear to possess the power of turning starch into sugar, but, if mixed with the submaxillary secretion, it accomplishes that transmutation with facility. The saliva, as obtained from the mouth, is therefore a mixture of the