252 NUTRITION 



the many various substances of the food and tissues are oxidized, so 

 general is the process and chemism so various. 



The fats and the carbohydrates from which water and carbon dioxide 

 (by oxidation of their carbon) largely come have already been sufficiently 

 described, as have also the means by which these excreta are transported 

 from the tissues and given out by the lungs and nasal passages through 

 the nostrils. (See the descriptions of katabolism above and the dis- 

 cussion of respiration in a preceding chapter.) 



The Sweat is the next most important of the means of excreting some 

 of the waste-products of katabolism. These are especially water, urea, 

 ammonia, kreatinin, sodium chloride, and phosphate, the inorganic and 

 ethereal sulphates, fats (Reid), and the inorganic salts, largely sodium 

 chloride. The amount of water excreted through the skin it is hard 

 to estimate from the discordant results obtained by experiment, for it 

 greatly varies not only at different times and under different internal 

 and external conditions, but also on different parts of the skin. The 

 "insensible" sweat is the more important in studying normal excretion, 

 yet every known means of measurement at once increases its output 

 many times. On the whole the average daily quantity is probably not 

 far from 1500 grams, the same as that of the urine, as will be recalled. 

 So far, then, as excreting water is concerned, these two means are equiva- 

 lent and the balance of their well-known and important reciprocal action 

 (one decreasing in amount as the other increases) is rendered more 

 perfect. 



The Feces, although consisting daily of several hundred grams of waste 

 material, excrete but a very small proportion of strictly katabolic product, 

 for they are in large part merely the refuse of the digestive apparatus. 

 It is here, however, that the systematic description of the feces properly 

 belongs. 



The quantity of the feces defecated daily by an adult living on an 

 ordinary mixed diet is about 160 grams, but on a vegetal diet the amount 

 may be three times as great, the difference being largely due, directly 

 or indirectly, to cellulose. The percentage of water is from 60 to more 

 than 80; the latter proportion belongs to the feces from a vegetal diet, 

 because peristalsis is then much more active and less time is allowed 

 for the water's absorption into the lymph- or blood-vessels of the colon. 

 The fecal color varies from light yellow to black, depending on the 

 amount of bile-pigments (stercobilin), iron salts, and sulphuretted 

 hydrogen present. The odor is due largely to skatol, but sulphuretted 

 hydrogen, fatty acids, and indol take part. The chemical reaction of 

 feces may be either acid or alkaline. 



The composition of the feces is of course exceedingly variable, for it 

 depends directly on the food ingested and not absorbed. Water makes 

 about three-quarters of its weight, and much of the solid material is 

 undigested and indigestible bits of food and dried digestive juices. The 

 undigested part of the food consists of bits of meat, fat-globules, and 

 carbohydrate; the indigestible fragments are of bone, ligaments, keratin, 



