form of gangrene may
occur in old persons, the term senile being here restricted to that
variety which results from arterio-sclerosis.

[Illustration: FIG. 20.--Senile Gangrene of the Foot, showing line of
demarcation.]

_Clinical Features._--The commonest seat of the disease is in the toes,
especially the great toe, whence it spreads up the foot to the heel, or
even to the leg (Fig. 20). There is often a history of some slight
injury preceding its onset. The vitality of the tissues is so low that
the balance between life and death may be turned by the most trivial
injury, such as a cut while paring a toe-nail or a corn, a blister
caused by an ill-fitting shoe or the contact of a hot-bottle. In some
cases the actual gangrene is determined by thrombosis of the popliteal
or tibial arteries, which are already narrowed by obliterating
endarteritis.

It is common to find that the patient has been troubled for a long time
before the onset of definite signs of gangrene, with cold feet, with
tingling and loss of feeling, or a peculiar sensation as if walking on
cotton wool.

The first evidence of the death of the part varies in different cases.
Sometimes a dark-blue spot appears on the medial side of the great toe
and gradually increases in size; or a blister containing blood-stained
fluid may form. Streaks or patches of dark-blue mottling appear higher
up on the foot or leg. In other cases a small sore surrounded by a
congested areola forms in relation to the nail and refuses to heal. Such
sores on the toes of old persons are always to be looked upon with
suspicion and treated with the greatest care; and the urine should be
examined for sugar. There is often severe, deep-seated pain of a
neuralgic character, with cramps in the limb, and these may persist long
after a line of demarcation has formed. The dying part loses sensibility
to touch and becomes cold and shrivelled.

All the physical appearances and clinical symptoms associated with dry
gangrene supervene, and the dead portion is delimited by a line of
demarcation. If this forms slowly and irregularly it indicates a very
unsatisfactory condition of the circulation; while, if it forms quickly
and decidedly, the presumption is that the circulation in the parts
above is fairly good. The separation of the dead part is always attended
with the risk of infection taking place, and should this occur, the
temperature rises and other evidences of toxaemia appear.

_Prophyl