472 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 



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 kincls 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 defaecation 

 is greatly prolonged on a purely flesh diet, and may only take place 

 at intervals of several days. The amount of faeces 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 faeces 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 haematin 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 Hasebrock, Ztsclir. f. physiol. Chem., Strassburg, 1888, Bd. xii. S. 148. 

 *Ztschr.f. Biol., Mlinchen, 1889, Bd. xxv. S. 264. 



