PARASITES AND SAPROPHYTES. 147 



satistactorily cultivated outside the body, or if it has, it has 

 always become somewhat altered in appearance. Sarcinae 

 have also been found in the bladder, in the air passages, 

 in the intestines, and even in the blood, but most of these 

 can be cultivated on nutrient gelatine. None of these 

 organisms are pathogenic so far as at present can be made 

 out ; they appear to be simply accidentally associated with 

 certain conditions, as in the case of dilated stomachs, in 

 which they are not always present, but in which they 

 frequently occur, apparently because the acid fermentation 

 that goes on in the accumulated contents renders them 

 specially favourable media for the development of these 

 sarcinae. The non-pathogenic parasitic bacteria of the mouth 

 were amongst the first organisms observed by Leeuwen- 

 hoek ; they are found on the gums, on the sordes covering 

 the teeth, and on the mucous membrane of the mouth. 

 The forms of these we shall mention later. Although most 

 of these are parasitic and non- pathogenic, one form, the 

 Leptothrix buccalis (an old term embracing probably several 

 species, and one not now very generally used), is also patho- 

 genic in the sense that it invades the teeth, and gives rise 

 to what is known as caries, or rotting of the teeth, but even 

 this has to obtain help from some of the other non-patho- 

 genic species found (some of the micrococci), which give rise 

 to the formation of lactic acid from the sugar of old food, 

 an acid that, combining with the lime salts of the teeth, 

 softens them, and so allows of the penetration of the lepto- 

 thrix form. 



Pasteur has suggested that other facultative saprophytes 

 are those met with in the alimentary tract, where he thought 

 they appeared to assist in carrying on the decomposition of 

 the nutrient materials, assisting, under certain circumstances, 

 the animal tissues of the body to carry on the process of 

 digestion without interfering in any marked degree with the 

 nutrition of the tissues of the host by absorbing any of the 

 material that could be utilized by these tissues for their own 

 nutrition. This, however, is now considered to be question- 

 able. The organism of cholera finding its way into the 

 alimentary canal, acts as a pathogenic parasite, but here 

 only from the fact that by its growth and metabolic activity 

 it gives rise to irritant toxic materials, which exert most 

 injurious effects both locally and constitutionally. 



