130 



THE EFFECTS OF PARASITES ON THEIR HOSTS. 



FIG. 78. Ureter, with ex- 

 crescences due to the presence 

 of Uietomum. 



Echinococcus of the liver and the Strongylus of the kidney affect tissue 

 metabolism by suppressing the biliary and urinary secretions ; and 

 the Trichinae, by extensive destruction of 

 muscular tissue, occasion more or less com- 

 plete paralysis. This paralysis disappears 

 some time after the infection, on account 

 of a new formation of the muscular tissue, 

 but no such renewal takes place after the 

 destruction of liver or brain tissue, so that 

 there the functional deviations are permanent, 

 and often increase to such a degree that death 

 is inevitable. It is true that in such cases the 

 death of the host is not always the direct con- 

 sequence of the first operations of the parasites, 

 but often results from secondary disturbances ; 

 and among these, besides the cachexial and 

 dropsical conditions often arising in the course 

 of time from chronic ailments, the slow 

 inflammations and dissolutions brought about 

 by the irregular circulation, caused by the 

 pressure on the larger blood-vessels, ought specially to be noted. 



Fortunately cases of this description are rare. Most of the paren- 

 chyma-worms exert a less dangerous influence, and many, such as the 

 Cysticercus cellulosce of the muscles, cause hardly any disturbance. 

 The result always depends upon circumstances, or, in other words, 

 in such a case as the present, upon the amount of pressure exerted and 

 the physiological importance of the affected organ. Thus the specific 

 nature of the parasite is quite irrelevant, so that Cysticercus cellulosce, 

 for example, which we have just described as harmless when inhabiting 

 the muscles, becomes a most dangerous parasite under certain circum- 

 stances and in certain situations, as in the brain or in the eye. 



In wide spaces, where the parasites multiply into larger masses, 

 their influence consists not so much in pressure as in a more or less 

 complete obstruction of the passages. The influence of this obstruction 

 on the health again depends principally on the relative importance 

 of the organ in the economy of the organism. Thus the occurrence 

 of Cysticercus in the anterior chamber of the eye, or in the vitreous 

 humour, causes blindness. Similarly the aggregation of masses of 

 worms (Stronrjylus s. Ryngamm trachealis) in the windpipe of birds, 

 and especially of our common fowls, often causes suffocation ; and the 

 intertwining of tape-worms and thread-worms in the human intestines 

 produces conditions which have a great resemblance to intussusception 

 of the intestine, or to strangulated hernia. If the interruption be not 



