igS 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



enough to pass easily through the capillary network but when they find 

 themselves traversing the capillaries of a muscle — particularly if it be one 

 of the more active muscles such as the diaphragm — they bore their way 

 out of the capillary and pass in amongst the muscle fibres. After 

 spending some time, it may be several days, migrating through the 

 muscle, the young worm curls itself up into a spiral and settles down in 

 a resting condition between the muscle fibres (Fig. 89), the connective 

 tissue round it reacting to its presence by enclosing it in a lemon-shaped 

 cyst about -4 mm. in length. 



Within its cyst the young Trichina may retain its vitality for many 



1 



mf 



Fig. 89. 

 Larval Trichinae encysted in muscle, x 26. m.f, Muscle fibres ; T, Trichinae. 



years but this is unusual and as a rule after a few months or years the 

 cyst undergoes calcification. 



No further development takes place unless the muscle containing the 

 Trichina is swallowed by a suitable animal. When this happens the 

 young worm is set free by the digestion of the cyst : it rapidly — within 

 from one to two days — attains to sexual maturity and the life-cycle is 

 started afresh. 



Pigs are particularly liable to become infected, probably through 

 eating the flesh of an infected rat, and then the infection is liable to be 

 conveyed to man by his eating insufficiently cooked pork or ham con- 

 taining the encysted worms. As the infection is liable to be a heavy 

 one, resulting in the presence of many millions of young worms within 

 the body, severe symptoms of disease (trichinosis) are produced — more 

 or less cholera-like symptoms during the intestinal stage, and high tem- 

 perature accompanied by se\-ere muscular pains during the period of 

 migration. 



