8o Causes of Disease. 



of one of the gastric flies (j^astrophilus sahttaris — so named from 

 this idea) ; it was thought that their presence in the stomach and 

 small intestine of the horse promoted the secretion of gastric 

 juice ; or it was believed that some parasites consumed superfluous 

 mucus. Although it is reasonable to suppose that some parasites 

 obtain their nutriment from the waste matter (as is true of the 

 infusoria and bacteria always present in the stomach in rumi- 

 nants) and take part in the breaking up the intestinal contents, 

 yet it is not to be concluded that the parasitic worms and arthro- 

 pods have any value whatever to their host. On the contrary, the 

 animal host, if rid of them, will tlirive equally and better. All 

 parasites exert from time to time some harmful influence, and 

 some are death-bearing guests. 



In the first place, many parasites are harmful to their host and 

 productive of' disease by their wandering in the body and their 

 movements. Undoubtedly the itching caused by the crawling para- 

 sites inhabiting the skin-surface, and often giving the host no 

 rest day or night, has some influence upon the health ; here may 

 be mentioned the sheep-lice, itch-mites and the oxyurides inhabit- 

 ing the rectum and causing continual tickling sensations, as per- 

 fect torments which often give rise to emaciation (due to the in- 

 crease of metabolism following the incessant restlessness from the 

 itching). Other parasites make their way through the tissues by 

 boring, and thus where any considerable number penetrate into 

 the organs cause tissue destruction and occasion haemorrhages and 

 inflammatory lesions (lesions produced by liver flukes, larval tape- 

 worms and lingiiatulidse). They may give rise to a great variety 

 of disturbances, just as any foreign bodies in a mechanical way, 

 by compression of tissues, or by obstruction and narrowing of 

 tubular passages, as sudden death by invading the brain, paralysis 

 by pressure on the s])inal cord, atrophy of the liver by growth in 

 its parenchyma, blindness by penetrating into the eye, rupture or 

 thrombosis of the larger blood vessels, respiratory interference and 

 suffocation by lodgment in the air passages and lungs, and intesti- 

 nal obstruction. The parasites adhering to the intestinal mucous 

 membrane by booklets or suctorial apparatus often produce re- 

 flexly marked irritative or perhaps nervous phenomena ; and, too, 

 a toxic influence is to be ascribed to the metabolic products of a 

 number of worms which may be reasonably thought to explain 

 the occurrence of nervous changes and general loss of flesh (as in 



