It is scarcely necessary to give a description of this species which
is sufficiently known; we shall content ourselves with indicating the
principal characteristics which distinguish it from the other vermin
living on the surface of the human body. The louse is of an oval form;
flattened, longer than the crab-louse; its head is very small; its
thorax is composed of three not very distinct rings; the abdomen is all
of one piece, rounded off on the sides; it is ash-colored, whereas the
crab-louse is entirely white. The lice which have been made use of in our
provings, were taken from the head of a healthy child of five years.
Even before undertaking this experiment we expected to derive from it
important results. We entertained the belief that nature pointed out to
us the louse as a specific for the hereditary psora, in which belief we
were strengthened by the fact that psorin develops the lice-malady in
healthy persons. And we are now prepared to affirm that we have found the
louse one of the most useful agents in diseases of children. We offer the
following pathogenesis with a sincere pleasure which none but those who
devote themselves to such patient investigations, can appreciate.[2]