28 ANIMAL PARASITES. 



gone through in that place. The mechanical injury done to the 

 skin by the mite, does not, however, appear to be the cause of the 

 eruption of the itch ; but, according to Gudden, this is to be found 

 in the circumstance that the mites, like other parasites, emit an 

 irritating fluid with their bite. Gudden supports this view by the 

 fact that we may penetrate beneath the skin with a needle, and 

 rub cinnabar into a canal thus formed and yet produce no itch- 

 like eruption, whilst by dropping in tincture of cantharides on 

 the mass formed of triturated mites, a little pain is produced at 

 first, and in the course of a day or two a slight exudation. More 

 remote proofs of this supposition may also be found, in my opi- 

 nion, in the phenomena following experiments in inoculation with 

 the matter of smallpox and syphilis. 



From Gudden's experiment of the transference of a mature, 

 impregnated female upon the healthy skin of an individual, the 

 following was ascertained as to the time of the process of develop- 

 ment. The mite dug its gallery and deposited its eggs. On the 

 ninth and tenth days, the individual experienced distinct gnawing 

 and pricking on particular parts of the hand, readily distinguishable 

 from the imaginary itching hitherto felt over the whole body, of 

 which the person had complained. Vesicles and papillae now rose 

 immediately, and these gradually diffused themselves in increasing 

 numbers along the arm ; from some of the vesicles which were just 

 rising Gudden was able to extract the rnite with the needle, and 

 in others, when the mite was already gone, he could detect the 

 gallery. The latter appears rounded off at the end, but sharply 

 bitten out at its entrance, and has, although only when superficially 

 examined, an accidental similarity with hair-follicles destitute of 

 hair, with the convolutions of sudorific canals, or especially with 

 the epidermic hoods of the cutaneous papillse. 



By experiments carried on simultaneously on distant parts of 

 the body, as well as in fresh cases of infection with itch, we may 

 perceive that every egg-gallery becomes the centre of a circle of the 

 disease, from which the young brood diffuses itself in radiating 

 galleries. When the disorder has lasted longer, the separate cir- 

 cles certainly become mixed, and it is then impossible to discover 

 them. 



The galleries vary greatly in size, and may be distinguished into 

 the long-known larger ones, which are recognisable by the naked 

 eye, scarcely raised above the level of the skin, and sensible to the 

 finger, and the smaller ones, which are scarcely visible. The larger 



