276 DIGESTIVE SYSTEM. 



quantity with the increase of substance in the canal. These 

 acids are generally the lactic and butyric acids, arising from 

 the fermentation and decomposition of the sugars and the 

 fats. 



Half-way through the large intestine, however, all digestion 

 and absorption cease : the tube contains only those matters 

 which are to be thrown off, ihefceces, in short. The faeces 

 have been wrongly considered as principally formed of 

 that part of the food which cannot be assimilated : if this 

 were true, if all the nourishment received can be absorbed, 

 there ought to be no fasces, and yet they appear, even in this 

 case. Thus the foetus, whose digestive tube is as yet empty, 

 immediately after birth expels fa?ces which are well known 

 under the name of meconium: the meconium is formed of 

 remains of epithelial cells, colored yellow by the bile, which, 

 not having yet become decomposed, preserves its natural 

 color. This explains why the principal product thrown off, and 

 of which the faaces are chiefly composed, consists of remains 

 of the desquamated epithelium : sometimes, even in the 

 adult, these remains alone form the substance of the faeces. 

 They appear either as entire or as mutilated globules of a 

 whitish color, variously tinged by the decomposed bile. 

 These epithelial remains somewhat resemble the fine scales 

 which fall from the cutaneous epidermis, but they are more 

 numerous and important than this; for we have seen that 

 the shedding of the epithelium is the fatal termination of the 

 series of phenomena of absorption, and that the principal 

 use of the bile is to regulate and to hasten its production. 



Those parts of the aliments and of the digestive fluids 

 which cannot be assimilated can only be classed as secondary 

 elements in the constitution of the laeces. Among these are 

 cholesterine and the coloring matter of the bile which are 

 precipitated when this fluid enters the intestine ; also fatty 

 substances, when ingested in too large quantities ; amylaceous 

 substances protected by too thick a covering of cellulose ; 

 and cellulose, in general, with its derivatives. Indeed vege- 

 table aliments contain the largest quantity of substances 

 which resist digestion, and the fa3ces of the herbivorous ani- 

 mals are, therefore, much more abundant than those of the 

 carnivora. Animal food, however, also contains elements 

 which long resist the influence of the digestive juices : thus 

 the horny growths of the epidermis (hair, nails, etc.), and 

 the yellow or elastic tissues (parts of tendons, of arterial 

 coats, etc.), are found in the fa3ces almost entire. 



