318 SOME COMMON INTERNAL PARASITES [CH. IX 



into the intra-muscular connective tissue 1 , and develop 

 around themselves a lemon-shaped cyst which in course of 

 time becomes calcareous, while the affected fibres undergo 

 degeneration. The symptoms at this stage are in some 

 respects similar to those of rheumatism. The natural host 

 of Trichina is the common rat, which is a dirty and 

 omnivorous feeder, and does not scruple to eat the dead 

 bodies of its brethren, and thus the infection is passed on 

 from generation to generation. Occasionally infected rats 

 are devoured by pigs, and thus man is exposed to the risk 

 of trichinosis. 



In tropical countries there are numerous other diseases 

 for which species of Nematodes are responsible. Inter- 

 mediate hosts have been discovered in some cases, notably 

 the mosquito as the introducer of Filaria sanguis hominis. 

 In others there is no intermediary, but infection is direct. 

 To one of these, Uncinaria (Anchylostoma) duodenalis, 

 special interest is attached from the fact that it was 

 introduced some seven or eight years ago into Dolcoath 

 Mine, Cornwall. This misfortune was doubtless brought 

 about by some miner returning infected from a tropical 

 country. Dolcoath Mine is some 3,000 feet in depth and 

 the temperature in it ranges from 75 F. to 90 F. The 

 conditions are therefore well suited to the parasite. The 

 male is from 8 to 11 mm., the female from 10 to 18 mm. 

 long. They inhabit and become sexually mature in the 

 first portions of the small intestine. The eggs develop to 

 a certain stage within the parent and are evacuated with 

 the faeces of the host. Incubation only takes place in a 



1 Hertwig and Graham, Miinchener med. Wochensch. XLII. 1895. 



