802 PAEASITIC DISEASES. 



There is no doubt in my mind but that the ova and young 

 parasites taken up with the food, in the first place gain access from 

 the alimentary canal into the circulation, and are conveyed into 

 the lung-substance, where they are deposited, the parasites when 

 mature piercing the tissues and entering the bronchial tubes, and 

 there cause the irritation symptomatic of the disease, whilst those 

 remaining encysted in the lung cause little or no inconvenience 

 (see fig. 42). It is very true that in many instances the para- 

 sites are found fully matured in the digestive canal, and doubt- 

 less the conclusions of Dr. Crisp are due to this fact. We can, 

 however, easily understand that the heat and moisture of the 

 stomach are quite sufficient to cause the ova to hatch and some 

 of the embryos to mature in the intestinal canal, whilst other 

 embryos pierce the intestinal wall and are taken into the 

 circulation, and mature only when they have been deposited in 

 their proper habitat ; and a few may become fully developed 

 in the blood. I have on two occasions seen the parasite in 

 the cavities of the heart and in the blood-vessels ; and I think 

 this fact conclusively points out the correctness of this view of 

 their migration. 



The tenacity of life in the young strongyle is very great. 

 Ercolani found that they showed signs of life on being moistened 

 after drying for thirty days, and at other times after having 

 been immersed in spirits of wine at 30°, or in a solution of alum 

 and corrosive sublimate. — (Gamgee.) 



The number of embryo worms in the lungs of one sheep is 

 very great ; if to these we add those hatched in the digestive canal, 

 we can easily understand, when these are severally discharged 

 from the infected animal, how a pasture may become infected 

 with parasites and ova to such an extent as to infect a whole flock. 



Dr. Crisp says the disease is due to over-stocking, and especially 

 to the feeding of lambs off a second crop of clover after the first 

 crop has been consumed by sheep. The mere feeding of lambs 

 with the second crop of clover, after the first has been con- 

 sumed by sheep, would not of itself be sufficient to cause an 

 outbreak of the lamb disease; but when we consider that a 

 very large number of sheep are infested with the parasites, from 

 which they seem to suffer uo harm, we can easily understand 

 how, by the expulsion of some of these from the affected a 

 pastm^e may become fouled by the parasites and their ova. 



