402 INTERNAL PARASITES. 



. . . They are frequently met with in couples ; two individuals 

 forming almost a right angle, and adhering so intimately that they 

 may be preserved in this condition in alcohol " {Neumann). Their 

 presence, when they are in considerable numbers, sometimes 

 causes death by exhaustion and diarrhoea. 



Palisade worms seem to gain entrance into the body of the horse, 

 in the form of embryos, which the animal swallows in the water ho 

 drinks, and in the damp forage he eats. Having arrived at their 

 intended place of residence, they penetrate through the mucous 

 membrane of that part, underneath which each worm forms a 

 tumour (or cyst), ais a result of the products of the inflammation 

 caused by their presence. These cysts or temporary nests, which 

 may be seen or felt on the surface of the intestine when it is opened 

 after death, vary in size from that of a pin's head to that of a hazel 

 nut. The cysts and the presence of the worms in them do not 

 appear to cause any serious disturbance to the animal's health. The 

 immature worms in the cyst vary from -03- of an inch to ^ of an 

 inch in length. After having passed through a stage of develop- 

 ment in their respective cysts, the worms leave them ; some going 

 into the intestine, to attach themselves to its mucous membrane, 

 to assume the adult form, and to produce eggs, which are expelled 

 with the dung. These eggs are oval in shape, are about -^\-^ of 

 an inch long, and about half as broad. They become hatched in 

 water or damp dung in from three to eight days under a tempera- 

 ture of from 59° to 77° F. The embryos thus produced are about 

 g^Q of an inch in length. They have a blunt head and a thread- 

 like tail. 



The iminature worms which do not issue from the cysts directly 

 into the intestine, get into arteries (chiefly those of the great 

 mesenteric, which is the supplier of blood to all the intestines, 

 with the exception of a portion of the rectum), and are then 

 liable to set up inflammation in these vessels, with the result of 

 a sac being formed by the dilatation of the wall of the artery. 

 These sacs vary in size from that of a pea to that of a man's head. 

 The wall of the sac becomes thickened, and the sac becomes more 

 or less filled with a fibrous deposit formed by the inflammatory 

 exudation and by clotted blood. There is a passage for the blood 

 left in the fibrous deposit, which may extend for some distance over 

 the internal surface of the artery, either towards the heart or away 

 from it. The sac and its contents are called an aneurism ; and the 

 fibrous deposit, a thrombus. Immature, pink-coloured worms, of 

 a half to one inch in length, and averaging about nine in number, 

 are, as a general rule, found in the centre of the clot. Portions of 

 the blood-clots formed by these aneurisms are apt to break off, 

 and, on being carried along in the blood-current, to block up the 



