ZOOLOGICAL DISTRIBUTION OF DISEASE. 275 



elephantiasis is so frequent that they are manifestations 

 of the same disease. 



According to the admirable researches of Dr. Manson 

 the chief facts connected with this disease are the 

 following : 



The adult worm is probably taken into the alimentary 

 canal of man in drinking-water. From the stomach 

 it bores its way into the thoracic duct or some 

 lymphatic vessel, and is subsequently joined by one of 

 the opposite sex. Here they may live for years, dis- 

 charging their embryos into the lymph stream, to become 

 distributed by the blood current over the body. 



One of the most remarkable phenomena connected 

 with filarial disease is that the embryos disappear from, 

 and reappear in, the general blood stream at certain 

 periods of the day. Under ordinary conditions the blood 

 of an affected individual presents no filariae between the 

 hours of nine a.m. and six p.m. ; after six p.m. they begin 

 to make their appearance and increase in numbers until 

 midnight, at which hour as many as 260 filaria have 

 been counted in a single drop of blood : from this hour 

 they gradually diminish in numbers and at nine o'clock 

 in the morning they cannot be detected in the blood. 

 During the night, and whilst the filarial migration is at 

 its height, mosquitoes visit the body, and Manson has 

 identified the female, a particular variety of Culex which, 

 by the structure of its oral appendage, abstracts blood 

 from the filariated individual, be it man or beast. As 

 the culex selects eight o'clock in the evening as the feed- 

 ing hour it necessarily follows that the embryo filariae 

 are taken into the stomach of the mosquito, and Manson 



