GLOSSARY. PARASITE. 603 



Little is yet known of the parasitic fungi that form on raucous 

 membranes, or in the organs covered by mucous or serous mem- 

 branes ; they seem, however, to be secondary in character, and 

 of much less pathological importance than those of the outward 

 surface. 



The nature of the alteration produced in man and animals on 

 the outer surface of the body by parasitic fungi is the infiltration 

 or destruction of hairs and epithelial structures by the sporules of 

 the fungus, which, in the progress of their development uniting, 

 grow into a branched mycelium. 



There is reason to think that the number of species of fungi 

 capable of this kind of existence is extremely limited. 

 . There must be some peculiar condition of nutrition which dis- 

 poses a plant or animal to become the abode of such fungi some 

 failure in the vital power of the parts of the organic tissue affect- 

 ed, so that they cannot any longer carry on efficiently the processes 

 of life, whence, like dead organic matter, they become a prey to new 

 though inferior living organisms. 



Such a notice as this must confine itself to partial views of the 

 whole subject of parasitic organisms. The mites or acari which 

 connect themselves with certain states of the surface in living 

 animals should not be passed by. From an early period it had 

 been suspected, and is now fully settled, that itch in the human 

 body depends on the presence of an acarus or mite, named now 

 the Sarcoptes scabiei humana. Different species of Sarcoptes have 

 been described ; thus there is a Sarcoptes equi, a Sarcoptes suis, a 

 Sarcoptes canis. The Sarcoptes from the human body do not live 

 on other animals in general ; the Sarcoptes equi may live on man, 

 developing the form of mange observed in the horse. Parasites 

 that do not penetrate the skin, but simply hold on by it, have been 

 named Dermatodectes ; and those which only pierce the cuticle 

 in search of nourishment have been called Symbiotes. The Der- 

 matodectes equi cannot live except on the horse ; and the Symbiotes 

 equi cannot induce the same disease as in the horse, in either the 

 ox, sheep, pig, or dog. Mange is less common in the ox than in 

 the horse, yet two distinct parasites are described in the ox, one 

 belonging to the genus Dermatodectes, the other to the genus Sym- 

 biotes. 



The parasite on which scab in sheep depends is not an Acarus or 

 Sarcoptes, as is usually said, but a Dermatodectes that is to say, it 

 does not pierce the skin or burrow in the skin. 



The mange of the pig, however, is due to a Sarcoptes. It ap- 

 pears that the mange of pigs may be communicated to man ; but 

 it is not certain that the Sarcoptes suis can live on the horse, ox, 

 sheep, or dog. 



The mange in dogs depends on a Sarcoptes; it may live for a 



