TRANSMISSION OF TROPICAL DISEASES. 15 



were the first to prove it — that the flea transmits the disease 

 from rat to rat, and there can be no doubt from rat to man. 

 How does it effect this ? This has only recently been explained. 

 When the flea sucks blood containing bacilli into its stomach, 

 the bacilli rapidly multiply and to such an extent that they 

 actually block the oesophagus or rather the proventriculus. 

 The result of this block is that the flea can get no more blood 

 into its stomach and so feels thirsty. In order to relieve 

 its thirst it goes on sucking by means of its pharyngeal pump, 

 but as the blood cannot pass the block some of it regurgitates, 

 owing to the increased pressure back into the wound, but it 

 is now contaminated by the bacilli in the blocked proventriculus 

 and so the wound sets infected. 



FlLARIASIS. 



One of the most noteworthy conditions included under 

 this heading is Elephantiasis. This is a condition, as you 

 know, in which there is great swelling, e.g., of the legs due to 

 obstruction in the lymphatics caused by quite delicate worms 

 about 1J-4 inches long. These worms give birth to larval 

 forms, which are about one-third mm. long, and they find 

 their way from the lymphatics into the blood stream where 

 they move freely about. When a mosquito sucks blood 

 containing these larvae, the latter pass — how has not been 

 followed — into the thoracic muscles of the mosquito and 

 there come to rest. After a time they become active again 

 and are now found in the proboscis of the mosquito, not in the 

 tube which conducts the blood up, or in the tube which conducts 

 the saliva down, but in the muscular substance of the labium, 

 the rod in the groove of which the tubes and cutting lancets 

 lie. They are thus in a cul-de-sac, and it is thought that 

 they emerge by rupture of a membrane which is stretched 



