ous cure, as is observed in some of the ordinary traumatic sacs.
This history of development is moreover supported by the observation
that proximal ligature of the artery usually converts an arterio-venous
aneurism into an aneurismal varix. The process is no doubt favoured by
cleanness and small size of the perforation, moderation in the amount of
primary haemorrhage, the tone and resistance of the surrounding tissues,
special points in the circulatory force and condition of the blood, and
the possibility of maintaining the part at rest after the injury.

Aneurismal varix, when pure, was evidenced by the presence of purring
thrill and machinery murmur alone. In none of the cases I saw was pain
or swelling of the limb present. In one popliteal varix, slight
varicosity of the superficial veins of the leg was present, but it was
not certain that the development of this was not antecedent to the
injury, as the patient did not notice it until his attention was drawn
to its existence. In none of the cases under observation in South Africa
had enough time elapsed for sufficient dilatation of the artery above
the point of communication to give rise to any confusion from this cause
as to the presence of a sac.

When an arterio-venous sac has once formed, clinical observation shows
that the general tendency is towards extension in the direction of least
resistance. This direction of course varies with the situation of the
aneurism, and also with the nature of the wound track.

Speaking generally the direction of least resistance in a typically pure
perforation is towards the vein. Initial flow of blood from the wounded
artery is naturally favoured towards the potential space afforded by a
canal occupied by blood flowing at a lower degree of pressure. The
partial collapse of the vein dependent on the wound in its wall also
probably helps in determining the initial flow in its direction.
Examples are afforded by the carotid aneurisms (cases 10, 11, and 14),
and here it must be borne in mind that the outer limits of the cervical
vascular cleft are those least likely to offer resistance to extension
of the sac. In each the aneurisms mainly occupied the exit segment of
the track; this is the general rule, as in the case of external
haemorrhage, and is determined by the same cause.

The latter rule however finds exceptions when the entry segment is so
situated as to cross a region of lesser resistance, and case 12
illustrates this p