CH. IX] OF DOMESTIC ANIMALS 317 



A species more important, from the effects produced by 

 it in man, is the microscopic Trichina spiralis, the cause of 

 the disease trichinosis. This form is parasitic throughout 



Fig. 54. Trichina spiralis, encysted amongst muscular fibres. 

 Highly magnified. After Leuckart. 



its entire life-history and has no free stage of existence, 

 such as is not infrequent among other Nematodes. The 

 disease is contracted by eating " underdone " pork infested 

 by the encysted miniature Trichince which have lodged 

 themselves in the muscle fibres of the pig, where they are 

 capable of remaining alive for many years. The larva? 

 are liberated from their cysts by the digestive fluids of the 

 unfortunate human being and in about three days become 

 sexually mature. They then resort to the tubular glands 

 of the small intestine 1 . Here a brood of larvae, amounting 

 to as many as 1,000 individuals from each female, is, in 

 about another four days, viviparously produced. These bore 

 their way through the lining of the intestine into the 

 blood vessels of their host, occasioning severe symptoms, 

 and at times even death. If death is not caused the 

 larvae are swept along by the blood-stream, and eventually, 

 aided by their own wriggling movements, pierce their way 



1 Geisse, Miinchener med. Wochensch. XLII. 1895 ; and Zool. Centralbl. 

 iv. 1897. 



