174 HEALTH AND DISEASE 



entirely exceptional. The fluke is not one of the worms which finds a 

 host in that animal under ordinary circumstances. 



Another parasite which is found in the liver of the horse is the cystic 

 stage of an extremely small tape-worm found in the intestines of the 

 dog, the Tmiiia echinococcus. The worm, when fully grown, is on an 

 average about \ inch in length, and never exceeds ^ inch, but in its cystic 

 (hvdatid) stage it is one of the largest which exists. 



The Cysticercus echinococcus is found frequently in the liver, and 

 occasionally the lungs, of cattle and sheep; the cysts varying in size from 

 that of a gi'ape to that of an orange, as a rule, but now and then they 

 are found of an enormous bulk. Each cyst contains a fluid in which 

 are found floating a number of tape- worm heads, myriads of which are 

 observed growing on the interior of the cyst. In one form of the parasite 

 small cysts, or daughter-vesicles as they are called, are found abundantly 

 in the fluid. This peculiarity has given rise to a division of the parasite 

 into two classes: — 



1. The Ecliinococcus cdtricipariens, in which the secondary vesicles 

 exist. 



2. The Echinococcus scolicipariens, in which they are replaced by the 

 small spots on the membrane, and in the fluid the tape-worm heads 

 (^Scolices). 



The presence of these hydatids in the liver and other organs of animals 

 is often not attended with any indications of disease, even when the liver 

 is so filled with the cysts as apparently to replace the normal structure. 



On the serous membrane of the chest and abdomen small wandering 

 echinococcus cysts are occasionally found. There is also a nematode worm 

 (Filaria) which has been found in the peritoneal and pleural cavities 

 of the horse, ass, and mule. It does not appear to have been recoi'ded, 

 however, among the parasites of the horse in this country. 



In the circulatory system of the horse, parasites are occasionally en- 

 countered, as the Surra parasite, found in the blood of horses in India, 

 and the embryos of the Strongylus armatus and Strongylus tctracanthus, 

 which locate themselves in the anterior mesenteric artery, and cause a 

 well-marked aneurism. It is comparatively common in the ass. Parasites 

 in the nerve-centres, or in the organs of special sense of the horse, are 

 extremely rare. There is one case recorded by Woodger of the presence 

 of a hydatid in the brain of a horse. In this case the animal suffered 

 from the same kind of giddiness and tendency to turn in one direction 

 as is known to be characteristic of a sheep similarly affected with 

 hydatid in the brain, and there are a few cases reported of the discovery 

 of the embryos of the armed Strongylus in the blood-vessels of the brain. 



