Experiment 46. Dilute one ounce of milk with ten times its volume of water. Add cautiously dilute acetic acid until there is a copious, granular-looking precipitate of the chief proteid of milk (caseinogen), formerly regarded as a derived alb.u.men. This action is hastened by heating.

Experiment 47. Saturate milk with Epsom salts, or common salt. The proteid and fat separate, rise to the surface, and leave a clear fluid beneath.

Experiment 48. Place some milk in a basin; heat it to about 100 F., and add a few drops of acetic acid. The ma.s.s curdles and separates into a solid curd (proteid and fat) and a clear fluid (the whey), which contains the lactose.

Experiment 49. Take one or two teaspoonfuls of fresh milk in a test tube; heat it, and add a small quant.i.ty of extract of rennet. Note that the whole ma.s.s curdles in a few minutes, so that the tube can be inverted without the curd falling out. Soon the curd shrinks, and squeezes out a clear, slightly yellowish fluid, the whey.

Experiment 50. Boil the milk as before, and allow it to cool; then add rennet. No coagulation will probably take place. It is more difficult to coagulate boiled milk with rennet than unboiled milk.

Experiment 51. Test fresh milk with red litmus paper; it should turn the paper pale blue, showing that it is slightly alkaline. Place aside for a day or two, and then test with blue litmus paper; it will be found to be acid. This is due to the fact that lactose undergoes the lactic acid fermentation. The lactose is converted into lactic acid by means of a special ferment.

Experiment 52. Evaporate a small quant.i.ty of milk to dryness in an open dish. After the dry residue is obtained, continue to apply heat; observe that it chars and gives off pungent gases. Raise the temperature until it is red hot; allow the dish then to cool; a fine white ash will be left behind. This represents the _inorganic matter_ of the milk.

Experiments with the Sugars.

Experiment 53. Cane sugar is familiar as cooking and table sugar. The little white grains found with raisins are grape sugar, or glucose. Milk sugar is readily obtained of the druggist. Prepare a solution of the various sugars by dissolving a small quant.i.ty of each in water. Heat each solution with sulphuric acid, and it is seen to darken or char slowly.

Experiment 54. Place some Fehling solution (which can be readily obtained at the drug store as a solution, or tablets may be bought which answer the same purpose) in a test tube, and boil. If no yellow discoloration takes place, it is in good condition. Add a few drops of the grape sugar solution and boil, when the mixture suddenly turns to an opaque yellow or red color.

Experiment 55. Repeat same experiment with milk sugar.

Chapter VI.

Digestion.

128. The Purpose of Digestion. As we have learned, our bodies are subject to continual waste, due both to the wear and tear of their substance, and to the consumption of material for the production of their heat and energy. The waste occurs in no one part alone, but in all the tissues.

Now, the blood comes into direct contact with every one of these tissues.

The ultimate cells which form the tissues are constantly being bathed by the myriads of minute blood-vessels which bring to the cells the raw material needed for their continued renewal. These cells are able to select from the nutritive fluid whatever they require to repair their waste, and to provide for their renewed activity. At the same time, the blood, as it bathes the tissues, sweeps into its current and bears away the products of waste.

Thus the waste occurs in the tissues and the means of repair are obtained from the blood. The blood is thus continually being impoverished by having its nourishment drained away. How, then, is the efficiency of the blood maintained? The answer is that while the ultimate purpose of the food is for the repair of the waste, its immediate destination is the blood.[19]

129. Absorption of Food by the Blood. How does the food pa.s.s from the cavity of the stomach and intestinal ca.n.a.l into the blood-vessels? There are no visible openings which permit communication. It is done by what in physics is known as _endosmotic_ and _exosmotic_ action. That is, whenever there are two solutions of different densities, separated only by an animal membrane, an interchange will take place between them through the membrane.

To ill.u.s.trate: in the walls of the stomach and intestines there is a network of minute vessels filled with blood,--a liquid containing many substances in solution. The stomach and intestinal ca.n.a.l also contain liquid food, holding many substances in solution. A membrane, made up of the extremely thin walls of the blood-vessels and intestines, separates the liquids. An exchange takes place between the blood and the contents of the stomach and bowels, by which the dissolved substances of food pa.s.s through the separating membranes into the blood.

[Ill.u.s.tration: Fig. 46.--Cavities of the Mouth, Pharynx, etc. (Section in the middle line designed to show the mouth in its relations to the nasal fossae, the pharynx, and the larynx.)

A, sphenoidal sinus; B, internal orifice of Eustachian tube; C, velum palati; D, anterior pillar of soft palate; E, posterior pillar of soft palate; F, tonsil; H, lingual portion of the pharynx; K, lower portion of the pharynx; L, larynx; M, section of hyoid bone; N, epiglottis; O, palatine arch ]

This change, by which food is made ready to pa.s.s into the blood, const.i.tutes food-digestion, and the organs concerned in bringing about this change in the food are the digestive organs.

130. The General Plan of Digestion. It is evident that the digestive organs will be simple or complex, according to the amount of change which is necessary to prepare the food to be taken up by the blood. If the requisite change is slight, the digestive organs will be few, and their structure simple. But if the food is varied and complex in composition, the digestive apparatus will be complex. This condition applies to the food and the digestion of man.

[Ill.u.s.tration: Fig. 47.--Diagram of the Structure of Secreting Glands.

A, simple tubular gland; B, gland with mouth shut and sac formed; C, gland with a coiled tube; D, plan of part of a racemose gland ]

The digestive apparatus of the human body consists of the alimentary ca.n.a.l and tributary organs which, although outside of this ca.n.a.l, communicate with it by ducts. The alimentary ca.n.a.l consists of the mouth, the pharynx, the sophagus, the stomach, and the intestines. Other digestive organs which are tributary to this ca.n.a.l, and discharge their secretions into it, are the salivary glands,[20] the liver, and the pancreas.

The digestive process is subdivided into three steps, which take place in the mouth, in the stomach, and in the intestines.

131. The Mouth. The mouth is the cavity formed by the lips, the cheeks, the palate, and the tongue. Its bony roof is made up of the upper jawbone on each side, and the palate bones behind. This is the _hard palate_, and forms only the front portion of the roof. The continuation of the roof is called the _soft palate_, and is made up of muscular tissue covered with mucous membrane.

The mouth continues behind into the throat, the separation between the two being marked by fleshy pillars which arch up from the sides to form the soft palate. In the middle of this arch there hangs from its free edge a little lobe called the uvula. On each side where the pillars begin to arch is an almond-shaped body known as the tonsil. When we take cold, one or both of the tonsils may become inflamed, and so swollen as to obstruct the pa.s.sage into the throat. The mouth is lined with mucous membrane, which is continuous with that of the throat, sophagus, stomach, and intestines (Fig. 51).

132. Mastication, or Chewing. The first step of the process of digestion is mastication, the cutting and grinding of the food by the teeth, effected by the vertical and lateral movements of the lower jaw.

While the food is thus being crushed, it is moved to and fro by the varied movements of the tongue, that every part of it may be acted upon by the teeth. The advantage of this is obvious. The more finely the food is divided, the more easily will the digestive fluids reach every part of it, and the more thoroughly and speedily will digestion ensue.

The act of chewing is simple and yet important, for if hurriedly or imperfectly done, the food is in a condition to cause disturbance in the digestive process. Thorough mastication is a necessary introduction to the more complicated changes which occur in the later digestion.

133. The Teeth. The teeth are attached to the upper and lower maxillary bones by roots which sink into the sockets of the jaws. Each tooth consists of a _crown_, the visible part, and one or more fangs, buried in the sockets. There are in adults 32 teeth, 16 in each jaw.

Teeth differ in name according to their form and the uses to which they are specially adapted. Thus, at the front of the jaws, the incisors, or cutting teeth, number eight, two on each side. They have a single root and the crown is beveled behind, presenting a chisel-like edge. The incisors divide the food, and are well developed in rodents, as squirrels, rats, and beavers.

Next come the canine teeth, or cuspids, two in each jaw, so called from their resemblance to the teeth of dogs and other flesh-eating animals. These teeth have single roots, but their crowns are more pointed than in the incisors. The upper two are often called eye teeth, and the lower two, stomach teeth. Next behind the canines follow, on each side, two bicuspids. Their crowns are broad, and they have two roots. The three hindmost teeth in each jaw are the molars, or grinders. These are broad teeth with four or five points on each, and usually each molar has three roots.

The last molars are known as the wisdom teeth, as they do not usually appear until the person has reached the "years of discretion." All animals that live on gra.s.s, hay, corn, and the cereals generally, have large grinding teeth, as the horse, ox, sheep, and elephant.

The following table shows the teeth in their order:

Mo. Bi. Ca. In. In. Ca. Bi. Mo.

Upper 3 2 1 2 | 2 1 2 3 = 16 | } = 32 Lower 3 2 1 2 | 2 1 2 3 = 16

The vertical line indicates the middle of the jaw, and shows that on each side of each jaw there are eight teeth.

134. Development of the Teeth. The teeth just described are the permanent set, which succeeds the temporary or milk teeth.

The latter are twenty in number, ten in each jaw, of which the four in the middle are incisors. The tooth beyond on each side is an eye tooth, and the next two on each side are bicuspids, or premolars.

The milk teeth appear during the first and second years, and last until about the sixth or seventh year, from which time until the twelfth or thirteenth year, they are gradually pushed out, one by one, by the permanent teeth. The roots of the milk teeth are much smaller than those of the second set.

[Ill.u.s.tration: Fig. 48.--Temporary and Permanent Teeth together.

_Temporary teeth:_ A, central incisors; B lateral incisors; C, canines; D, anterior molars; E, posterior molars

_Permanent teeth:_ F, central incisors; H, lateral incisors; K, canines; L, first bicuspids; M, second biscuspids; N, first molars ]

The plan of a gradual succession of teeth is a beautiful provision of nature, permitting the jaws to increase in size, and preserving the relative position and regularity of the successive teeth.

[Ill.u.s.tration: Fig. 49.--Showing the Princ.i.p.al Organs of the Thorax and Abdomen _in situ_. (The princ.i.p.al muscles are seen on the left, and superficial veins on the right.)]