PRACTICAL WORK ON CHAPTER II.

1. Fit an indiarubber bag on the end of a glass tube, pass the tube through the stopper of a bell-jar. Place the bell-jar on the plate of an air-pump with the rubber bag in the jar and partly filled with air. When air is extracted from the jar the

rubber bag becomes inflated by air pressing in down the tube.

This illustrates the inflation of the lungs when the chest is enlarged by depression of the diaphragm and by elevation of the ribs.

glass belljar India-rubber

bag

Ө

plate

of air pump

2. Fit the indiarubber stopper of a bell-jar with a bent glass tube, to which a short length of indiarubber tubing is attached. Place the bell-jar to stand in a large basin containing water to the depth of several inches. Place a lighted taper in the jar--it burns as in ordinary air. Take it out and replace the stopper. Inhale the air from the jar through the tube. Water will rise as air is taken out and if there is sufficient water in the basin the jar will be filled with water when the whole of the air is extracted. Breathe the same air back into the bell-jar, causing the water to descend On testing with a taper it will be found that this air will not support combustion.

Fig. 12.-APPARATUS FOR ILLUSTRATING MECHANISM OF

RESPIRATION.

3. To prove the presence of moisture in expired air, breathe on the surface of a cold mirror.

4. Place some limewater in two test glasses. Blow ordinary air by means of a pair of bellows through the limewater in one, and breathe expired air through the other by means of a glass tube. The large amount of CO2 present in expired air will be very apparent by the milkiness produced. Cf. page 17.

5. Place a little Condy's Fluid in water. Breathe through this; the organic matter present in the expired air decolourises the Condy's Fluid.

6. Place some smouldering brown paper in a lamp chimney. Blow across the top with a pair of bellows, the lessening of pressure causes an up-draught which is shown by the motion of the smoke. This illustrates the action of the wind as a ventilating agent.

7. Place a lighted candle at the bottom of a cylinder (open at the top only). In a short time the candle flame is extinguished.

Take a T-piece made of metal which will fit into the top of the cylinder and divide the opening into two parts. The candle on being relighted will continue to burn.

The currents of air set up may be shown by means of smoke. Heated air passes up one side of the T-piece and fresh air down the other.

CHAPTER III.

FOOD-DIETS.

Uses of Food. These may be classified as follows:

:

1. To keep up the heat of the body. The food is first digested and assimilated, it is then oxidised and reduced to simpler forms in the tissues, and is finally excreted in the form of water, carbon dioxide, urea, and some indigestible matter. As a result of the chemical combination between the oxygen and the elements of the food, heat is produced.

2. To produce the energy which results from chemical actions in which food is concerned. This energy performs the work of the body, which may be divided into two kinds, (a) internal work, which consists in keeping the different organs automatically at work, and (b) external work, such as walking, running, or other muscular exertion.

3. To repair and renew the wasted tissues, caused by the constant wear and tear of the body.

4. To form new tissue in the process of growth of young people.

Classification of Foods. All foods are composed of certain substances called food stuffs or proximate principles. Some foods contain representatives from practically all the proximate principles, while others contain one only, or mainly one, with very small quantities of one or two others. The ultimate constituents of foods are the chemical elements of which they are composed. These are chiefly carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus.

Foods are classified according to their source into:(1) inorganic foods, which include water and the various mineral salts, and (2) organic foods, i.e. those of animal or vegetable origin. The organic foods are further subdivided

into those which contain nitrogen and those which do not. Those containing nitrogen are called nitrogenous or proteid, and those with no nitrogen are called non-nitrogenous foods. Non-nitrogenous foods are further subdivided into hydrocarbons (a misnomer) or fats, carbohydrates or sugars and starches, and the vegetable acids.

The following table shows clearly the division of foods into food stuffs.

Water stands second only to oxygen among the necessaries of life. About 70 per cent. of the body consists of water. It is necessary as a food, and also helps digestion by dissolving the digested food. In fact the greater part of food can only be absorbed by the body after it is in solution. Water is also necessary in order to maintain the fluidity of the blood, which contains about 80 per cent. of water, and to assist in the removal of waste matters, especially urea, which is excreted in solution in the

urine.

on.

The average person needs from 3 to 5 pints of water daily in order to supply the loss that is continually going About one-third of this is usually taken with the solid food, leaving about 3 pints of water to be drunk per day. In many foods the percentage of water is large, as shown by the following table :

Green vegetables 90-95 per cent. water.

These include chloride of sodium, or common salt, chloride of potassium, phosphates of potassium, calcium, and magnesium, and salts of iron. They are essential constituents of food. Thus common salt is the source of the hydrochloric acid present in the gastric juice of the stomach, and which is necessary for digestion. It is also the source of the sodium in bile salts, and is found in every fluid and tissue of the body. The calcium or lime salts are necessary to build up the skeleton: they are contained in most foods, especially milk and cheese. Phosphorus is an indispensable constituent of bone and brain, and the salts of iron are necessary to supply the iron present in the haemoglobin of the red corpuscles.

NITROGENOUS FOODS OR PROTEIDS.

Nitrogenous foods are conveniently arranged in three classes as follows:

The True Proteids.

Albumin constitutes nearly the whole of the solids in the white of eggs and one-third of those in the yolk, the remainder being fat. The average composition of albumin may be taken to be-Carbon, 54 per cent.

with small traces of phosphorus. The other members of the group of true proteids have practically the same composition. Albumin is, without doubt, the most important of all the food substances. It is the essential constituent of that substance, called protoplasm, which is described as the physical basis of life. It is soluble in water, precipitated by alcohol, and coagulated by heat.

Fibrin, together with serum albumin and serum globulin, is present in blood-plasma or, speaking more accurately, a substance is present in blood-plasma which forms fibrin under certain conditions, thereby causing the blood to clot.

Myosin, or muscle albumin, constitutes the bulk of dead muscle. It is coagulated by heat, like albumin, but differs from that substance by being insoluble in water and readily soluble in a 10 per cent. solution of common salt. From this solution myosin may be obtained as a flocculent white precipitate by adding it to a large quantity of distilled water.

Casein is the chief nitrogenous constituent of milk, which also contains some albumin. It is not coagulated by heat, but rennet (pepsin and acid) or free acids readily cause coagulation.

Gluten is the nitrogenous constituent of wheat flour; its elements are present in the seeds of most cereals, and, like albumin and myosin, it is coagulated by heat. The result, therefore, of cooking any food containing either of these three nitrogenous principles will be to coagulate the proteid constituent and render it insoluble in water or dilute acids.