128 THE EFFECTS OF PARASITES ON THEIR HOSTS. 



hardly be distinguished from the cysts of the parenchyma-worms, 

 which have been described (p. 19). The soft adjacent tissue begins 

 at the same time to disappear, 1 and is finally completely destroyed, 

 after more or less remarkable histological changes. We know, for 

 instance, that the substance of the brain is gradually destroyed by 

 the growth of the bladder-worm developing within it, and that the 

 liver atrophies more and more in consequence of the ever-increasing 

 pressure of Echinococcus, or of Distomum hepaticum, which inhabits 

 the bile ducts. 2 



If the pressure of the growing parasites restrict the supply of 

 blood to the infected organ, or in any other way affect its supply of 

 nutriment, the destructive work will of course advance even more 

 rapidly. In such cases the whole of the organ disappears, leaving 

 hardly any traces of its existence. Thus, in animals whose kidneys 

 are infected by Eustrongylus gigas the organ is ultimately only repre- 

 sented by insignificant sickle-shaped thickening (Fig. 76) surrounding 

 the pelvis, which has been distended by the worm into a sac-like 

 form. 3 



Further, even parasites of smaller size may, under certain circum- 

 stances, achieve the same results. The tiny embryos of the Trichincc, 

 for instance* soon cause disintegration of the sarcous elements of the 

 muscular fibres which they perforate, so that the latter are transformed 

 into granular sheaths. At first these sheaths have quite the same 

 shape as the normal sarcolernma- sheaths ; but afterwards, as the worms 

 grow, an enlargement is formed around their resting-place, and this, 

 after the disappearance of its ends and other changes, becomes a 

 " TVic/traa-capsule " (Fig. 77). 4 



The eggs of parasites may produce the same effects as the worms 

 themselves, if deposited in the interior of narrow channels, or in the 

 parenchyma of the organs, as in the case of Distomum (Bilharzia) 

 hcematoHum. The venous branches underlying the mucous membrane 

 of the ureter in which this worm deposits its ova cause, through the 



1 Gb'ze tells (" Verauch einer Naturgesch., u. s. w.," p. 234) of "a very lean and 

 wretched looking " rat, of which the liver was so much riddled with Cysticerci that, " on 

 account of the great number of bladders, hardly any of the tissue could be seen. " Another 

 very similar case is also given, loc. cit. p. 243. 



2 In this way the bile ducts of the rabbits are changed into more or less closed cysta, 

 through the influence of the continuously developing and multiplying Psorospermiae. A 

 similar result may be observed in the LieberkUhnian glands of frogs, when infested by 

 Nematodes and other parasites. According to Waldenburg (Mailer's Arckiv f. Anat. u. 

 Physiol., p. 195, 1862), the blood-vessels of the frog occasionally become worm-cysts, 

 through aneurismal distention and sacculation. 



3 Hence earlier observers thought the seat of the worm was in the interior of the 

 kidney. 



* See Vol. II. ; also Leuckart, " Untersuchungen uber Trichina spiralis," 2d Ed., p. 53. 



