tood, and an art when this
knowledge can be applied to determine its location and exact nature.
Science presents the general principles of practice; art detects among
the characteristic symptoms the differential signs, and applies the
remedy. Da Costa aptly remarks: "No one aspiring to become a skillful
observer can trust exclusively to the light reflected from the writings
of others; he must carry the torch in his own hands, and himself look
into every recess."

The critical investigation of symptoms, with the view of ascertaining
their signs, is essential to successful practice. Without closely
observing them, we cannot accurately trace out the diagnosis, and a
failure to detect the right disease is apt to be followed by the use of
wrong medicines.

General diagnosis considers the surroundings of the patient as well as
the actual manifestations of the disease. It takes into account the
diathesis, _i.e._, the predisposition to certain diseases in consequence
of peculiarities of constitution. We recognize constitutional
tendencies, which may be indicated by the contour of the body, its
growth, stature, and temperament, since all these facts greatly modify
the treatment. Likewise the sex, age, climate, habits, occupation,
previous diseases, as well as the present condition, must be taken into
account.

Auscultation, as practiced in detecting disease, consists in listening
to the sounds which can be heard in the chest.

Percussion consists in striking upon a part with the view of
appreciating the sound which results. The part may be struck directly
with the tips of the fingers, but more generally one or more fingers of
the other hand are interposed between the points of the fingers and the
part to be percussed, that they, instead of the naked chest, may receive
the blow; or, instead of the fingers, a flat piece of bone or ivory,
called a _pleximeter_, is placed upon the chest to receive the blow.

Latterly, improved instruments greatly assist the practitioner of
medicine in perfecting this art. The _microscope_ assists the eye, and
helps to reveal the appearance and character of the excretions,
detecting morbid degenerations; _chemistry_ discloses the composition of
the urine, which also indicates the morbid alterations occurring in the
system; by percussion we can determine the condition of an internal
organ, from the sound given when the external surface is percussed; the
ear, with the aid of the _stethoscope_, de