INFECTION BY MEANS OF WATER. 161 



bility of such being the case must in nowise be overlooked. 1 We 

 are, of course, dealing only with the ordinary course of events, for 

 vegetables are as liable as flesh to an accidental association with 

 either adult or embryonic parasites. We have, indeed, every reason 

 to suppose that along with raw vegetable food certain thread-worms 

 (especially Tricliocephalus and Oxyuris) very commonly find their way 

 into man in the form of eggs, containing embryos. 



It seems very doubtful whether there be any internal worms 

 which bore their way into man, like the itch-mite or female chigoe. 

 The only example which could be cited of such a mode of infection 

 was the Guinea-worm, and in regard to this form also the theory has 

 been disproved by the above-mentioned observations of Fedschenko. 



The itch-mites and chigoes behave like those parasitic insect- 

 larvae which occur in superficial organs, like the larvae of (Estrus in 

 the sub-epidermal connective tissue, or the larvae of Musca, found in 

 the ear passages, or in the nasal cavity, &c. The latter differ, indeed, 

 in this that it is the free-living mother which places them in these 

 various situations. 2 When the grubs are found living in the intestine, 

 the introduction of eggs or larvae has taken place mostly by means 

 of cold meat, cheese, &c., just as we saw to be the case with the in- 

 testinal worms ; or in some instances the eggs may have come directly 

 from the mother, having been laid on the lips or tongue during sleep. 



Cases of parasites actively forcing an entrance into man are then 

 rare and restricted to a few forms. The true parasites the parasites 

 fear ifyyfiv exhibit no instance of it, and thus we may conclude 

 that the accidental introduction of eggs and immature stages is, after 

 all, ly far the most frequent and most constant source of the human 

 Entozoa. 



Every one exposed to one or other of these modes of introduction, 

 runs the risk of being infected with worms of various kinds, accord- 

 ing to circumstances. 



OCCURRENCE AND DISTRIBUTION. 



One often hears of a certain liability to helminthiasis varying 

 with age, sex, and even nationality. 



1 The statement of Ercolani, that the Anguillula and Rhabditis of plants become 

 genuine parasites after their transference into the intestine of animals, rests upon a 

 delusion. [On the other hand, the fact that the introduction of Distomum hepaticum is 

 effected from water-plants or marsh -plants, on which its Cercarise are encapsuled, has 

 been made extremely probable through the investigations of Thomas and myself. R. L.] 



2 As a rule, these insects are only induced to lay their eggs by some evil-smelling 

 excretion. On this point see v. Frantzius, Virchow's Archiv f. pathoL Anat., Bd. xliii., 

 p. 98, 1868. 



L 



