THE BACTERIA OF THE NORMAL HUMAN BODY. 155 



probably serves as a safeguard against the introduction of 

 pathogenic germs into the intestines. That is particularly 

 important in the case of the spirillum of cholera, which is 

 excessively sensitive to the action of acids. Nevertheless 

 many bacteria are able to reacrTthe intestines uninjured, as 

 the acidity of the gastric juice does not reach its height 

 until some hours after eating. Such bacteria will be those 

 which are most resistant and those which form spores. In 

 the intervals when hydrochloric acid is absent from the 

 stomach, lactic acid appears. It is formed from carbohy- 

 drates by a large number of species of bacteria. In con- 

 ditions of fermentation, sarcina ventriculi and yeasts may 

 be present in large numbers; in the healthy stomach they 

 occur in much smaller numbers. 



The intestine of the infant in whom feeding has become 

 well established was found by Escherich to contain two 

 principal species of bacteria in the lower part of the in- 

 testine the bacillus coli communis, in the upper part the 

 bacillus lactis aerogenes. More recently it has been shown 

 that the stools of milk-fed infants, and to a less extent of 

 adults, contain large numbers of anaerobic bacilli, which 

 stain by Gram's method (bacillus bifidus Tissier, bacillus 

 acidophilus Moro). These bacteria have not been fully 

 studied. 1 



The number of bacteria in a milligram of human fecal 

 matter has been estimated at from seventy thousand to 

 thirty-three million. It is estimated that about one-third of 

 the fecal matter of adults consists of bacteria. The small 

 intestine of adults has been found by different observers to 

 contain very different species. The majority of these ap- 

 pear to have been introduced from the mouth in food or 

 water. The bacillus coli communis, however, occurs in- 

 variably in health not only in the intestine of man, but also 



1 Metchnikoff, " Les Microbes Intestinaux," Bulletin dc I'Institut Pas- 

 teur, May 15 and 30, 1903. 



14 



