Repeated and sudden raising of the arterial tension, as a result, for example, of violent muscular efforts or of excessive indulgence in alcohol, plays an important part in the causation of aneurysm.  These factors probably explain the comparative frequency of aneurysm in those who follow such arduous occupations as soldiers, sailors, dock-labourers, and navvies.  In these classes the condition usually manifests itself between the ages of thirty and fifty—­that is, when the vessels are beginning to degenerate, although the heart is still vigorous and the men are hard at work.  The comparative immunity of women may also be explained by the less severe muscular strain involved by their occupations and recreations.

Syphilis plays an important part in the production of aneurysm, probably by predisposing the patient to arterio-sclerosis and atheroma, and inducing an increase in the vascular tension in the peripheral vessels, from loss of elasticity of the vessel wall and narrowing of the lumen as a result of syphilitic arteritis.  It is a striking fact that aneurysm is seldom met with in women who have not suffered from syphilis.

#Varieties—­Fusiform Aneurysm.#—­When the whole circumference of an artery has been weakened, the tension of the blood causes the walls to dilate uniformly, so that a fusiform or tubular aneurysm results.  All the coats of the vessel are stretched and form the sac of the aneurysm, and the affected portion is not only dilated but is also increased in length.  This form is chiefly met with in the arch of the aorta, but may occur in any of the main arterial trunks.  As the sac of the aneurysm includes all three coats, and as the inner and outer coats are usually thickened by the deposit in them of connective tissue, this variety increases in size slowly and seldom gives rise to urgent symptoms.

As a rule a fusiform aneurysm contains fluid blood, but when the intima is roughened by disease, especially in the form of calcareous plates, shreds of clot may adhere to it.

It has little tendency to natural cure, although this is occasionally effected by the emerging artery becoming occluded by a clot; it has also little tendency to rupture.

#Sacculated Aneurysm.#—­When a limited area of the vessel wall is weakened—­for example by atheroma or by other form of arteritis—­this portion yields before the pressure of the blood, and a sacculated aneurysm results.  The internal and middle coats being already damaged, or, it may be, destroyed, by the primary disease, the stress falls on the external coat, which in the majority of cases constitutes the sac.  To withstand the pressure the external coat becomes thickened, and as the aneurysm increases in size it forms adhesions to surrounding tissues, so that fasciae, tendons, nerves, and other structures may be found matted together in its wall.  The wall is further strengthened by the deposit on its inner aspect of blood-clot, which may eventually become organised.