ON THE ACARUS, OR INSECT OF ITCH AND MANGE. 
Bj/ Professor Hertwig, of Berlin. 
[Continued from p. 378]. 
The number of acari found on a mangy horse differs in differ¬ 
ent seasons, and also depends on the time that the animal has 
been infected, and the extent of the infection. From the be¬ 
ginning of spring until the autumn these insects are to be found in 
the greatest numbers on mangy horses. In the colder seasons 
their numbers are perceptibly diminished, and they sometimes 
seem to have almost vanished ; but this, however, they never com¬ 
pletely do. Their habits are the same as with the acari of the 
liuman being. They penetrate through the skin, and form under 
it little passages of different lengths and in different directions; 
partly to obtain nourishment, and partly in order to deposit their 
eggs. 
In warm weather and sunshine they may often be found on 
the surface of the skin in the mangy sores, on the sound skin, 
and even on the hairs. In the spots at which they have pierced 
through the skin a knotty elevation of the cuticle is found, 
which gradually becomes softer, and soon separates itself from 
the true skin (in the same manner as in scalds), and dries into 
a scaly scab, which, with the hair, is easily loosened from the 
cutis. This always takes place in from seven to ten days, first 
only in a very small spot, but soon afterwards in ])atches of 
greater width. 
If a small piece of the upper skin through which the acari 
have pierced is submitted to the power of a good microscope, 
there will be found on and sometimes under the true skin small 
passages, the diameter of which is hardly so great as that of the 
finest hair, and at the end of these passages are the acari. This, 
however, will not be perceptible if the disease has existed more 
than four or five days, as, after that, the outer skin becomes 
withered, and ceases to exist. On the true skin these passages 
have more the appearance of grooves or furrows. 
Under the dead cuticle there exists an exhalation of coagu¬ 
lated matter, which usually is first yellow, slightly glazed, and 
then grey. It soon dries into a horny, scaly crust, and as fast as 
one layer falls oft* it is succeeded by anotlier. This produces 
considerable itching, and, in order to ease this, the animal rubs 
and bites the mangy place until he often draws blood. The skin 
