DISEASES OF THE SKIN AND SUB-TISSUES. 278 
animals, with a few observations on the dog-flea (pulex canis). 
Bcabies is a pestiferous disease, whether it affects the horse, the 
ox, the sheep, swine, or poultry, inflicting a loss not easily esti- 
mated; hence the maxim of every intelligent farmer is, to avoid. 
With him “prevention is better than cure,” and, therefore, his 
grand desideratum is to guard against contagion. Sheep are, per- 
haps, more subject to it than any of the other animals, arising as 
much from the vature of their skins and coats as from the fecun- 
dity of the acarus ovis, and the greater vicissitudes of the weather 
to which they (the sheep) are exposed. Certain parts of the body 
are more liable to be affected than others; and so is an unhealthy 
skin than a healthy one. Indeed, it has been said that an unheal- 
thy skin will itself produce scabies (?), but this conclusion does 
not appear to be well founded; for a disease dependent upon the 
presence of living parasites can never arise spontaneously, but 
must be effected by contagion, either by means of their eggs, or 
the insect in some other stage of its existence. 
Now, from what has just been said, it will appear obvious that 
cleanliness, a healthy skin and state of the body, and a separation 
ffom foul animals and ground, are the means necessary to avoid 
contagion. The truth of this will, perhaps, be better understood 
if we first review the important distinctions which Mr. Simonds 
makes between the habits of the acarus scabiei of the human bods 
and the acari of our domestic animals, the former burrowing in 
the skin, but the latter living on its surface, clinging to the skin, 
hair, or wool with their trumpet-shaped, vesicular-cushioned feet, 
to prevent their being thrown off by the animal when shaking or 
nibbling itself. Hitherto distinctions of this kind have been over- 
looked, writers generally concluding that the acari of qnadrupeds 
burrowed in the skin like those of man, thus proving the little 
use which had been made of the microscope in examining the 
former, as it shows them to be incapable of living in the skin, 
from the configuration of their bodies. Indeed, to have made 
similar acari for naked skins as for those covered with hair, wool, 
or feathers would have been an oversight on the part of Nature; 
while the fact that the acarus scabiei will not live on the horse, 
nor acarus equi on man, or acarus ovis on the ox, or acarus bovis 
on the sheep, and so on, proves that greater differences than the 
mere configuration of the animal structure exist, all pointing to 
the above means as necessary, in every case, to avoid so great 9 
