sulphate of iron. The tissues of old animals give a firmer jelly than those of young ones, as they contain less water chemically combined.

Elasticin, or the matter of yellow, fibrous tissue, is remarkably insoluble. Boiling for thirty hours at a temperature of 320° (as in Papins' digester), produces a brown solution, which does not set on cooling. The matter of the elastic layers of cornea, capsule of lens, and other transparent membranes, including probably cellulose, have similar properties.

Keratin occurs in hair, nails, hoofs, whalebone, &c., and is said to contain 6 or 8 per cent. of sulphur. It dissolves in alkalies and mineral acids. Dried mucus contains a similar body, mucin. The last group are the colouring matters.

Hæmatin (C44 H22 N3 O6 Fe., Mulder) is the red pigment of the blood. It dissolves in alcohol, containing hydrochloric acid, or alkaline solutions, which brighten its colour. It absorbs oxygen, becoming a brighter red, and evolves carbonic acid. The matter containing iron which the spleen yields, and hæmatoidin from old extravasions, are modifications of hæmatin.

Biliphæin, or bile-pigment, is brown, but changes by oxidation to green. It is abundant in biliary calculi, and in jaundice, in urine, blood, the fluids of eye, and sometimes in saliva and sweat. It is probably formed from blood pigment, as bilifulvin, a variety of bile pigment, is identical with hæmatoidin. The green matter discharged when the liver is excited by mercurials is similar to hæmatin, and the effect of this medicine in destroying the colour of blood is well known.

Urophæin, or urine pigment, is little known. Harley finds it always contains iron, and believes it to be altered hæmatin. The difference of tint will not be objected when one remembers the change of colour blood undergoes in being absorbed from a bruise.

Melanin, or the colouring matter of hair, choroid, skin,

and some morbid products, is another nitrogenized substance. Indican, a blue precipitate, identical with indigo, has been found in the urine, especially in Bright's disease, and in pus.

There are other substances in animal chemistry indefinately termed "extractive matter," alcoholic or aqueous, according to the solvent. Among those hereafter will probably be discovered many bodies of interest.

QUESTIONS FOR EXAMINATION.

JUNIOR.

1. Enumerate those elements which compose the human body and their symbols.

2. What chemical uses does water fulfil in man's body?

3. Say what you know regarding the physiological chemistry of common salt.

4. Explain the change which starch undergoes when ingested. 5. State the composition of the fats.

6. Enumerate the so-called protein compounds, and the situations in which each occurs.

7. What substances analogous to fibrin and casein can be obtained from the vegetable kingdom?

8. What are the properties of gelatin ?

SENIOR.

1. What arrangements of the chemical constituents of the human body have been made?

2. What tissues contain phosphate of lime in greatest abundance, and how is it conveyed to them?

3. Under what circumstances have accidental elements been found in the human system?

4. Enumerate the varieties of sugar found in the human body, and the tests of their presence.

5. Give the characteristics of the albuminoids.

6. State the differences, physical and chemical, between albumen and fibrin.

7. Contrast the chemical features of white and yellow fibrous tissues.

8. What chemical components of the human body contain iron most abundantly?

DIGESTION.

EVERY vital action is attended with destruction of tissue, the materials of which are then excreted. Now, this loss is made good by food, which is prepared by the process, or rather series of processes, we call digestion. Food. The amount of matter which is constantly changing in the human body is enormous, as may be learned from the fact that about 800lb of solid food, 1,500 lb of fluids, and about 800 tb of oxygen, which is gaseous food, are annually ingested. The total, over 3,000 lb, or about a ton and a-half, exceeds the weight of the body twenty times, and we surmise from this that the whole system is renewed in considerably less than seven years, the period assigned by the older physiologists. Before describing the nature of animal food, it may be stated that it is all produced originally by plants, which elaborate their tissues from inorganic matter. The functions of those two classes of organic beings are well contrasted in the following table arranged after Prof. Galloway:

From the soil, then, through plants and animals, there

is always occurring a circulation of matter.

Animals

therefore derive their food either from the vegetable or animal kingdoms, or from both, and are thus divisible into herbivorous, carnivorous, and omnivorous, or more correctly amphivorous, to which latter class, as we shall find, man belongs. Domestication, has however, the effect of changing the food of animals; for instance, the cat can be altered from a carnivorous creature to one living wholly on vegetable food, the substitution being attended with remarkable change of habit, the fierce tiger-like wild-cat becoming our harmless fireside companion. On the other hand, however, herbivorous animals can be seldom brought to eat flesh.

Prout divided food into four groups-aqueous, saccharine, oleaginous, and albuminous. The first, with some other substances, belongs to the inorganic kingdom, the others to the animal or vegetable. The second and third groups are usually placed together as nonnitrogenous, respiratory, or calorific food, and the last is termed nitrogenized, plastic, or histogenetic. We shall, for convenience, adopt their division by Liebig, into respiratory and plastic-premising that all the pabulum for respiration and animal heat is not afforded by the food just ingested, a considerable portion being produced by the combustive destruction of tissues. In describing these groups of food, we must refer for chemical details to the last chapter.

I. Respiratory Food includes starch, sugar, the pectine bodies, fats, alcohol, and the aliments which contain them. Starch, as such, cannot be assimilated, but is changed into sugar mainly by saliva. Sugar may be still further changed into fat. It is thus that Negroes become so fat during the sugar season, when they consume it plentifully. The same race disproves the popular error that sugar injures the teeth, for theirs are remarkably white and sound; indeed, by producing lactic acid, it would cleanse the teeth, removing tartar, which is mainly composed of phosphate of lime. Moreover

Tomes found that teeth were not acted on by prolonged maceration in solutions of sugar.

Fats are plentifully partaken of by man, the more so the colder the climate is; and every one has heard of the appetite of the northern races for oily food. Sir John Franklin relates that he tried how much fat an Esquimaux boy could consume; after devouring 141b of tallow candles, he finished with a piece of fat pork, with which Sir John closed the experiment, feeling apprehensive for his store. Even children of five or six months old, influenced by hereditarily transmitted habit or imitation, will devour blubber with avidity, as witnessed during his residence in Greenland, by the late Sir Charles Gieseckè, Professor of Mineralogy in the Dublin Society. The rarity of phthisis in these climes may be due to this peculiarity in diet. The percentage of fat in some foods is very great, in others small-thus, pork, 30 to 50; mutton, 40; beef, 30; cheese, 25; veal, 16; herring, 6; goat's milk, 5·0; cow's milk, 3.5; human milk, 3; sole, 0.25.

Alcohol in some form enters into the diet of nearly all nations. Rice (arrack), milk (kumiss), honey (mead), juniper berries (gin), grapes (wine), cereal grains (whiskey, beers), are some of the substances which are used to obtain it by fermentation. Its formula is C4 H6 O2. Being so rich in carbon, it would appear to be suited for a heat-giving food, a view supported by Liebig, Moleschott, and other chemists, for the following reasons:—It is easily oxidised out of the body; those who use it take little other calorific food; and it is not rapidly or freely thrown off by pulmonary or cutaneous surfaces. If such views be correct, it economises the tissues, for after its ingestion less carbonic acid is evolved, and hunger does not return so soon, a fact which travellers are aware of. It quickens the circulation, which may account for its diffusing warmth, and excites the brain, so that no practical physician neglects to avail himself of its stimulant