We are thus led by the natural sequence of events to consider the composition of the faeces, a subject, like most of those already mentioned, of great interest in connection with our interpretation of the dietetic theories soon to be discussed.

It was formerly thought that the faeces consisted of nothing but food residuum, but it has now been abundantly proved that their composition is much more complex and is derived from several distinct sources. In the first place, the secretions of the stomach, pancreas, liver and intestine contribute an important share, and in this way bile acids, small quantities of pancreatic juice, hydrobilirubin, stercobilin, lecithin, cholesterol and some decomposed bile salts may all be found in association with dead epithelial cells from the mucous membrane. In addition, we find the remains of indigestible substances taken with the food, such as cellulose, keratin, mucin, chlorophyll, gums and resins, as also food residues which have escaped absorption, and actual undigested food when too great a quantity has been ingested, e.g., tendons, elastin, uncooked starch, various phosphates and salts of the alkaline earths, neutral fats, etc. Water is always present in variable proportions, depending upon the diet, but an average of 75 per cent. is usual, while bacteria of all sorts, and the products of the decomposition of food such as indol, skatol, phenol, fatty acids, haematin and insoluble soaps of calcium and magnesium are usually to be met with.

The composition of the faeces will vary according as cellulose is or is nob present in the diet. Even during fasting, faeces are still excreted which are similar to those found on a cellulose-free diet, consisting chiefly of digestive secretions; e.g., Cetti, the professional faster, voided 20 grams of faeces per day, equal to 3.47 grams of dried substance. In isolated loops of the intestine in living animals Hermann found after three weeks faecal substance weighing from 13 to 20 grams after drying.

Unless the food contains cellulose, there is little or no food residue, and therefore no protein, soluble carbohydrate, nuclein, nor connective tissue is to be discovered. On a concentrated cellulose-free diet, 70 per cent. of the faeces consist of water and the other 30 per cent. of fatty acid soaps, lecithin, a little neutral fat, mucin and nucleoprotein, but no protein nor starch. The ash is chiefly calcium phosphate.

All animals, however, require ballast of some kind in their food to provide an indigestible residue, whereby peristalsis may be stimulated and the essential faecal elements removed. For this reason carnivorous animals eat bones, granivorous birds sand and feathers, and the herbivorous animals enormous quantities of cellulose. Rabbits soon die on a cellulose-free diet, but when horn parings are substituted life is prolonged indefinitely. The weight of the dried faecal residue in man on a purely animal diet is from 13 to 28 grams, on a mixed diet from 30 to 40 grams, and on a purely vegetable diet as much as 74-115 grams, equal to from 300 to 400 grams when actually voided, and containing quite large quantities of protein and carbohydrate.

By adding cellulose to the diet, therefore, there is not only some food-residue, but because of this an increase in the other constituents of the faeces. There is a greater secretion of succus entericus because of the increased peristalsis due to irritation of the cellulose and the bacterial decomposition of the carbohydrates. Strassburger has demonstrated that quite one-third of the weight of dried faeces consists of bacteria, chiefly dead, and that 128,000,000,000,000 are evacuated daily.

Cellulose consists of the cell walls of plants, including their fibro-vascular bundles, but is usually regarded as indigestible or insoluble starch. So far as is known, no enzyme exists in man for its dissolution, but from 5 to 50 per cent. of it, depending upon the method of preparation of the food, is decomposed, probably by means of bacteria, carbonic acid and marsh gas being at the same time evolved. It has long been known from experiments on ruminants that from 60 to 70 per cent. of cellulose is utilised, and from 30 to 40 per cent. of the cellulose of sawdust and paper is absorbed by sheep when mixed and eaten with hay.

Varying proportions of the food-stuffs are recoverable from the faeces in accordance with their preparation and the method of their ingestion. Proteins are generally well absorbed when taken in the usual forms of flesh foods, eggs, milk, etc, there being not more than a loss of from 5-7 per cent. in a full diet of 100 grams of protein, or from .8 to 1.2 grams of nitrogen. On a coarse diet, where vegetables, fruit, and wholemeal bread are the chief nutrients, there is a loss of from 3 to 4 grams of nitrogen, equal to from 15 to 30 per cent. or more of the total protein. The loss, is always less on a suitable well-balanced mixed diet than on a one-sided diet.

In the case of vegetable protein-containing foods, a large proportion of the protein remains undigested by reason of its insoluble cellulose envelope, and hence cannot be absorbed. Voit has shown that quite 42 per cent. of the nitrogen present in a vegetarian's food was evacuated in the faeces, although we know there is practically no food-residue if the food be free from cellulose. These facts rather lend colour to the view that there is little fear of auto-intoxication on a meat or mixed diet in a healthy subject where moderation is observed and an efficient evacuation of the bowels occurs daily. It is to be noted that on a vegetarian diet there is on the contrary always some surplus of unabsorbed protein; but apart from the fact that it is claimed that vegetable protein is less putre-fiable than animal protein, and that most of the protein is encased in an insoluble covering of cellulose, intestinal stagnation is much less likely, and hence absorption of toxins most unusual.

Amongst the carbohydrates, sugar and dextrin are usually completely absorbed, but one or two grams of starch, equal to less than 1 per cent. of a mixed diet containing from two to four hundred grams of carbohydrates may be discovered.

It is estimated that of bread made from the finest flour 1.1 per cent, is lost, whereas 2.6 per cent. of that made from coarse flour, 7.4 per cent. of that made from wholemeal, and as much as 10.9 per cent. of that made from sour rye flour may be lost. Besides, in such cases fermentation is apt to arise, lower fatty acids are found in the colon, and these excite peristalsis. On a mixed diet it is calculated that from 1.2 per cent. of the carbohydrates is lost, and on a coarse diet the loss is as much as 5 per cent.

Where small quantities of fats are taken there is often a much greater loss, both proportionate and actual, than when large supplies are ingested. On an average, however, the loss is about 5 per cent.

In cases of chronic constipation the faeces are reduced in amount from 50 to 75 per cent., and this applies not only to the contained water, but to all the constituents, even including cellulose.

It is interesting to note, therefore, that practically none of the food-stuffs escape digestion except those entangled in the meshes of a cellulose envelope. On account of the greater peristalsis, when cellulose is present the faeces contain more fluid.

The following is a comparison of the faeces excreted in mixed and vegetarian diets respectively: -

Mixed diet.....

35 grams dry substance. 100 grams water.

Vegetarian diet ...

75 grams dry substance. 260 grams water.

Rubner states that the average fuel value of 1 gram of the dry organic substance of the faeces is 6.2 calories.