4 7 2 CHEMISTR Y OF THE DIGESTIVE PR O CESSES. 
formation of carbon-dioxide, methane, and ammonia ; but if air is present, 
neurin and muscarin are also formed in the process. 1 
COMPOSITION OF FAECES. 
Amount and consistency. — The consistency of the contents of the 
small intestine in the upper two-thirds to three-fourths of its length 
is fairly uniform, the amount of water absorbed in this part being 
approximately balanced by that added in the digestive fluids. But 
in the lower part of the small intestine the amount of water absorbed 
begins to exceed that secreted ; the intestinal contents become 
thicker, and the thin fluid, with lumps of solid, undigested, or 
partially digested food of various kinds floating in it, which is usually 
found in the higher part of the intestine, is replaced by a pasty 
or semi-solid mass. As this mass passes along the large intestine the 
process of absorption continues with increased intensity, and a large 
amount of water, together with anything it holds in solution of service 
to the economy, is removed. The residue, a complex mixture of various 
useless or unused material, usually acquires the consistency of a soft 
solid before the completion of the process, and is finally ejected from the 
rectum. The consistency of the faeces, as well as the amount excreted 
per diem, varies within wide limits, with the character of the food 
and the duration of its passage through the intestine. Even in the 
rectum the process of absorption goes on, and faeces retained here become 
dry and hard. The faeces passed on a vegetable diet, or on a diet con- 
taining a liberal allowance of vegetables, are both much softer {i.e. 
contain more water) and much greater in total quantity of dry solids 
than those on a meat diet alone. The increase in the quantity of solids 
is due to the vegetable food containing a much higher percentage of 
undigestible tissue. The softer consistency arises from the stimulation of 
the mucous membrane by the undigested remnants of the vegetable 
tissue, causing increased peristalsis, so hastening the transit through the 
intestine, and shortening the period of absorption. This stimulating 
action of vegetable food adds greatly to its value in a mixed diet. In 
consequence of the absence of this stimulus, the period of defalcation 
is greatly prolonged on a purely flesh diet, and may only take place 
at intervals of several days. The amount of fa'ces daily excreted by 
man on a mixed diet averages, according to Voit, 2 120 to 150 grms., 
containing 30-37 grms. of dried solids ; on a vegetarian diet, the 
average amount obtained was 333 grms., containing 75 grms. of dry 
material. 
Colour. — The colour of the fasces varies greatly, being mainly influ- 
enced by the nature of the food. On a diet of meat, the colour is dark 
brown to pitch black, due to haematm and to ferrous sulphide, formed 
by the action on haemoglobin-derivatives, of sulphuretted hydrogen 
generated by bacteria in the intestine. Administration of iron or 
bismuth salts produces a similar effect. A liberal allowance of bread, 
especially of the coarser varieties, in the food, gives rise to light yellow- 
coloured faeces. Fat, when eaten in greater quantity than the animal 
requires, is excreted with the faeces chiefly as fatty acids and soaps, and 
1 Hasebriick, Ztschr. f. pkysiol. Chem., Strassburg, 1888, Bel. xii. S. 148. 
2 Ztschr./. Biol.. Miinclien, 1889, Bd. xxv. S. 264. 
