ON THE CONTAGION OF ITCH OR MANGE. 353 
acarus in neither case being able to produce itch or mange, like 
what it has been pretended has been observed, in which the 
groove, as a matter of course, constituted the pathognomonic cha- 
racter of the disease. 
Here let us pause for a moment. 
Since the groove constitutes by itself the certain sign of itch, 
good observers never fail, in the case of contagion of itch from 
horse to man, to rely upon the presence of this groove for form- 
ing their diagnostic. If the groove be wanting, the patient is 
considered as having prurigo, eczema, &c., according to the 
predominant eruption, and is treated altogether differently. In 
a word, the communication of mange or itch from horse to man, 
to be susceptible of proof, demanded what in neither case could 
be present, since the acarus of the horse makes no grooves. 
We maintain, that the horse acarus is unable to make grooves 
under our epidermis, not merely because observation has shewn 
this to be the case, but from Nature having refused to it the 
organs requisite for such a function. The feelers of the human 
acarus and its mandibles are extraordinarily formed to incise and 
detach and raise the epidermis ; and the horny appendages 
issuing out of the dorsal line of its body are specially designed 
for its progress within the groove. On the other hand, the 
feelers and mandible of the equine acarus are exclusively formed 
for puncture and suction : it has no organ for facilitating its 
possible course underneath the epidermis. I have proved that 
this acarus — and such is likewise the opinion of Messrs. 
Delafond, Bouley, jun., and other veterinarians — lives sheltered 
underneath the hair, in the midst of epidermic pellicles and 
eruptions produced by it, without ever digging any furrows or 
grooves. 
Upon what has gone before we base the following proposi- 
tions : — 
1st. That no observation, confirmatory and irrefutable, of the 
communication of itch to man by the acarus of the horse has, up 
to this day, been published. 
2dly. That the cases admitted at the Hospital Saint-Louis 
during our sojourn there, as affected with itch communicated by 
the horse, when submitted to the examination of the microscope, 
have invariably shewn the grooves peculiar to the human itch. 
3dly. That the equine acarus, placed in precisely the same 
condition as the human acarus, have digged no furrows or 
grooves for the purpose of concealing themselves; that, in fact, 
it is not possible for the contagion of itch to pass from horse to 
man. Nor is it requisite for the communication of itch from 
the horse to our own persons that certain eruptions appear upon 
our skins. We shall consider this question of pathology 
further on. 
