400 INTEENAL PAEASITES. 



inches, and the total in one horse was estimated at more than a 

 million. 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. 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 he 

 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), as 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 ^ of an inch 

 to J of an inch in length. After having passed through a stage 

 of development 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 ^-g-^ of an inch long, and about half as broad. They become 

 hatched in water or damp dvmg in from three to eight days under 

 a temperature of from 59° to 77° F. The embryos thus produced 

 are about ^ of an inch in length. They have a blunt head and 

 a threadlike tail. 



The immature 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 tbe 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 



