POSSIBILITY OF SELF-INFECTION. 153 



Pentastomum tcenioidcs, reaching the exterior in the nasal mucus, are 

 transferred to us usually when the dog caresses us or licks our hands. 

 They may also find access directly to our food, such as bread and 

 salad, or may be deposited on vessels which we use in eating and 

 drinking. In the same way, and especially by dirty dishes or food, 

 are the eggs and embryos of the dog's tape- worm imported into us, 

 and this is all the easier since the eggs are not voided singly from their 

 host, but enclosed by the joints, which not only have a power of spon- 

 taneous motion, but may, in virtue of the glutinous surface of their 

 body, be carried about in many ways. The eggs of these animals also 

 frequently reach us through the medium of water used for drinking 

 or washing purposes. 



One must not, however, think that the existence of a parasitic 

 larval stage necessarily presupposes an importation from another 

 animal. It sometimes happens that man infects himself. This is 

 indeed constant in the case of Trichina, the embryos of which are, as 

 we know, born free in the intestinal canal of their host, whence they 

 wander without further change into the muscles, there to become the 

 well-known encapsuled worms. Man may also infect himself with 

 embryos of Tcenia solium, if he transfer the ripe joints or the eggs 

 alone into his stomach. I think it impossible, however, that any such 

 self-infection could take place directly in the intestine. This has 

 already been affirmed by Kiichenmeister, but the liberation of the 

 embryo presupposes a removal of the egg- capsule, and, so far as we 

 know, this never occurs before the eggs have passed through the 

 stomach. Experiment further contradicts Kiichenmeister's supposition. 

 In two cases where I succeeded in introducing eggs of Tcenia serrata 

 into the intestine of a young rabbit, by means of a fine syringe intro- 

 duced through an opening in the abdominal wall, the animal re- 

 mained free from bladder- worms. 1 If Kiichenmeister's view were 

 correct, then every one who suffers from tape-worm ought also to 

 suffer from bladder-worm, which, as is well known, is not the case. 

 But if, as often happens, we do find bladder-worms in those who suffer 

 or have suffered from tape-worm, that is because, when a tape-worm 

 is present, the introduction of joints or eggs is obviously an easier 

 matter than when they have to be brought from another man, not to 

 speak of other obvious possibilities, of which I mention merely these 

 that a patient harbouring a tape-worm may in vomiting easily run 

 a risk of transferring some part, or it may be only a few joints, of the 

 worm into his stomach ; and further, that in sleep, when the joints, 



1 This experiment, through which Kiichenmeister still ( ' ' Parasiten, " 2d ed., p. 115, 

 note) looks for the final decision of this question, has been long made by me, indeed 

 fifteen years ago ! 



