ly consists of two semilunar flaps attached to opposite sides of
the vessel wall, each flap having a small sinus on its cardiac side.
The distension of these sinuses with blood closes the valve and
prevents regurgitation. Valves are absent from the superior and inferior
venae cavae, the portal vein and its tributaries, the hepatic, renal,
uterine, and spermatic veins, and from the veins in the lower part of
the rectum. They are ill-developed or absent also in the iliac and
common femoral veins--a fact which has an important bearing on the
production of varix in the veins of the lower extremity.

The wall of _capillaries_ consists of a single layer of endothelial
cells.

HAEMORRHAGE

Various terms are employed in relation to haemorrhage, according to its
seat, its origin, the time at which it occurs, and other circumstances.

The term _external haemorrhage_ is employed when the blood escapes on the
surface; when the bleeding takes place into the tissues or into a cavity
it is spoken of as _internal_. The blood may infiltrate the connective
tissue, constituting an _extravasation_ of blood; or it may collect in a
space or cavity and form a _haematoma_.

The coughing up of blood from the lungs is known as _haemoptysis_;
vomiting of blood from the stomach, as _haematemesis_; the passage of
black-coloured stools due to the presence of blood altered by digestion,
as _melaena_; and the passage of bloody urine, as _haematuria_.

Haemorrhage is known as arterial, venous, or capillary, according to the
nature of the vessel from which it takes place.

In _arterial_ haemorrhage the blood is bright red in colour, and escapes
from the cardiac end of the divided vessel in pulsating jets
synchronously with the systole of the heart. In vascular parts--for
example the face--both ends of a divided artery bleed freely. The blood
flowing from an artery may be dark in colour if the respiration is
impeded. When the heart's action is weak and the blood tension low the
flow may appear to be continuous and not in jets. The blood from a
divided artery at the bottom of a deep wound, escapes on the surface in
a steady flow.

_Venous_ bleeding is not pulsatile, but occurs in a continuous stream,
which, although both ends of the vessel may bleed, is more copious from
the distal end. The blood is dark red under ordinary conditions, but may
be purplish, or even black, if the respiration is interfered with. When
one of the large veins in the n