170 A Description of the Thread-worm, Filaria immitis. 



follow them from a mucous tract (e. g. the iutestine) to a lacteal or 

 blood-vessel ; they follow the course of the circulation, growing on 

 the pabulum of the blood of the host, and easily passing with the 

 corpuscles through the capillaries ; soon their size unfits them to 

 traverse every viscus, and the minute capillaries of the lungs act as 

 a sieve to retain them in the venous circulation ; they copulate and 

 the females become fecund ; a young brood arises to continue on 

 the race, provided accidental causes, such as the mechanical blocking 

 up of important blood-vessels by the parent worm, do not determine 

 the death of the host. By this hypothesis the ingress of individuals 

 capable of arriving at maturity is explained, while the countless 

 hordes of young are rendered lucid only by the presence of one or 

 more parent worms within the vascular walls. These parent worms 

 after producing their progeny may possibly die and disintegrate, 

 and so account for their absence, or non-discovery, in hosts teeming 

 with the young brood. 



The question, however, — whence come the young which, entering 

 into the blood-vessels of the host, arrive at maturity ? is not clear. 

 Do they exist in the food or water which the dog feeds upon ? Are 

 they derived directly from the flesh of an infected animal fed upon, 

 or can they pass an intermediate state in water subsequently lapped 

 up by the animal ? Take for instance an infected dog dying and 

 disintegrating in a tank from which human beings and animals of 

 all descriptions slaked their thirst (no uncommon condition one 

 would think in Eastern countries), what would result to the im- 

 bibers ? Assuming the possibility of the young retaining vitality 

 in water, the impregnation of the water-drinkers by the young 

 worms, and their subsequent hfe history in the host as above 

 sketched, is highly suggestive, and supported by the experiments 

 with the trichina. Or, on the other hand, take for example a por- 

 tion of impregnated flesh taken in as food by human beings when 

 badly cooked, or eaten raw by any of the carnivora, is it not within 

 reason to assume the strong probabihty of infection ? Considering 

 the frequency with which the worms are found in dogs in China and 

 Japan, it is to be hoped that these doubtful points will in the early 

 future be cleared up by a few carefully-conducted experiments on 

 non-infected animals, and so by this means the possibility of any 

 human being carrying about within him swarms of loathsome micro- 

 scopic worms be averted. The identity also of the human worm 

 and canine embryo might be solved by the examination of the 

 living dog worms. 



It appears to me also that an accurate knowledge of the life 

 history of the Filaria immitis may throw much light on doubtful 

 points connected with the Guinea worm and congener forms. 



