gases, and which are produced by the various forms of bacteria, also come into the same category.

That the change or fermentation is produced by these organisms is shown by the fact that it occurs only when the organisms are present, and stops when they are removed or killed by a high temperature or

FIG. 16.-Typical forms of Schizomycetes (after Zopf): a, Micrococcus; b, macrococcus; c, bacterium; d, bacillus ; e, chlostridium; f, Monas Okenii; g, leptothrix; h, i, vibrio; k, spirillum; 7, spirulina; m, spiromonas; n, spirochete; o, cladothrix.

by certain substances (carbolic acid, mercuric chloride, &c.) called antiseptics.

The 'germ theory' of disease explains the infectious diseases by considering that the change in the system is of the nature of fermentation, and, like the others we have mentioned, produced by microbes; the transference of the bacteria or their spores from one person to another constitutes infection. The poisons produced by the growing bacteria appear to be either alkaloidal (ptomaines) or proteid in nature. The existence of poisonous proteids is a very remarkable thing, as no

chemical differences can be shown to exist between them and those which are not poisonous, but which are useful as foods.

There is another class of chemical transformations which differ very considerably from all of these. They, however, resemble these fermentations in the fact that they occur independently of any apparent change in the agents that produce them. The agents that produce them are not living organisms, but chemical substances, the result of the activity of living cells. The change of starch into sugar by the ptyalin of the saliva is an instance.

Ferments may therefore be divided into two classes:

1. The organised ferments-torulæ, bacteria, &c.

FIG. 17.-Bacillus anthracis, the agent that produces anthrax or splenic fever (Koch): A, Bacilli, mingled with blood corpuscles from the blood of guinea-pig, some of the bacilli dividing; B, the same after three hours' culture in a drop of aqueous humour. They grow out into long leptothrixlike filaments, which subsequently divide up, and spores are developed in the segments.

2. The unorganised ferments or enzymes-like ptyalin.

Each may be again subdivided according to the nature of the chemical change produced.

In digestion, the study of which we are just commencing, it is the unorganised ferments with the action of which we have chiefly to deal. The unorganised ferments may be classified as follows:

(a) Amylolytic-those which change amyloses (starch, glycogen) into sugars. Examples: ptyalin, diastase, amylopsin.

(b) Proteolytic-those which change proteids into proteoses and peptones. Examples: pepsin, trypsin.

(c) Steatolytic-those which split fats into fatty acids and glycerine. An example, steapsin, is found in pancreatic juice.

(d) Inversive-those which convert saccharoses (cane sugar, maltose, lactose) into glucose. Examples: invertin of intestinal juice and of yeast cells.

(e) Coagulative-those which convert soluble into insoluble proteids. Examples: rennet, fibrin ferment, myosin ferment.

Most ferment actions are hydrolytic-i.e. water is added to the material acted on, which then splits into new materials. This is seen by the following examples :

1. Conversion of cellulose into carbonic acid and marsh gas (methane) by putrefactive organisms.

(C6H10O5)n+nH2O= 3nCO2+3nCH1

2. Inversion of cane sugar by the unorganised ferment invertin :

C12H22O11+H2O = C6H12O6+C6H12O6

[cane sugar] [water] [dextrose]

[levulose]

A remarkable fact concerning the ferments is that the substances they produce in time put a stop to their activity; thus in the case of the organised ferments the alcohol produced by yeast, the phenol, cresol, &c., produced by putrefactive organisms from proteids, first stop the growth of and ultimately kill the organisms which produce them. In the case of the unorganised ferments, the products of their activity hinder and finally stop their action, but on the removal of these products the ferments resume work.

All ferments act best at a temperature of about 40° C. Their activity is stopped, but the ferments are not destroyed by cold; it is stopped and the ferments killed by too great heat. A certain amount of moisture and oxygen is also necessary; there are, however, certain micro-organisms that act without oxygen, and are called anaërobic in contradistinction to those which require oxygen, and are called aërobic.

The chemical nature of the enzymes or unorganised ferments is very difficult to investigate; they are substances that elude the grasp of the chemist to a great extent. So far, however, research has taught us that the ferments are either proteid in nature or are substances closely allied to the proteids.

LESSON VI

PEPTIC DIGESTION

1. Half fill four test tubes

A with water.

B with 0.2 per cent. hydrochloric acid.

C with 0-2 per cent. hydrochloric acid.

D with solution of white of egg (1 to 10 of water).

2. To A add a few drops of glycerine extract of stomach (this contains pepsin) and a piece of a solid proteid like fibrin.

To B also add pepsin solution and a piece of fibrin.

To C add only a piece of fibrin.

To D add a few drops of pepsin solution and fill up the tube with 0-2 per cent. hydrochloric acid.

3. Put all four tubes into the water-bath at 40° C., and observe them from time to time.

In A the fibrin remains unaltered.

In B it becomes swollen, and gradually dissolves.

In C it becomes swollen, but does not dissolve.

4. Examine the solution in test-tube B.

(a) Colour some of the liquid with litmus and neutralise with dilute alkali. Acid-albumin, syntonin, or parapeptone is precipitated.

(b) Take another test-tube, and put into it a drop of 1-per-cent. solution of copper sulphate; empty it out so that the merest trace of copper sulphate remains adherent to the wall of the tube; then add the solution from test-tube B and a few drops of strong caustic potash. A pink colour (biuret reaction) is produced. This should be carefully compared with the violet tint given by unaltered albumin.

(c) To a third portion of the fluid in test-tube B add a drop of nitric acid; albumoses or propeptones are precipitated. This precipitate dissolves on heating and reappears on cooling.

5. At the end of the lesson these three tests should be repeated with the digested white of egg in test-tube D.

THE SECRETION OF GASTRIC JUICE

The juice secreted by the glands in the mucous membrane of the stomach varies in composition in the different regions, but the mixed gastric juice, as it may be termed, is a solution of a proteolytic ferment called pepsin in a saline solution, which also contains a little free hydrochloric acid.

The gastric juice can be obtained during the life of an animal by

n

m

FIG. 19.-A pyloric gland from a section of the dog's stomach (Ebstein): m, Mouth; n, neck; tr, a deep portion of tubule cut transversely.

FIG. 18.-A cardiac gland from the dog's stomach (Klein): d, Duct or mouth of the gland; b, base or fundus of one of its tubules; on the right the base of a tubule is more highly magnified; c, central cell; P, parietal cell.