The size of an aneurism is variable, like its shape, but in general the true aneurism rarely exceeds the size of an egg (Jaccoud). Beyond this size one or more of the coats give way, and the aneurism becomes false, in which condition it may grow as large as an adult's head if the patient lives long enough to allow such development. Balfour refers to two rare forms of aneurism—the intravalvular, which is situated within the aortic valves and above the ventricle, and the intervalvular, which is still more rare, and is situated between the valves themselves. The symptoms of these aneurisms are merely those of valvular lesion, and therefore present no differential points for diagnosis.

ETIOLOGY.—Local weakness of the aorta submitted to sudden strain is unquestionably the most frequent cause of aneurism. It is rare to find an aneurism in an otherwise healthy aorta, and some authors go so far as to assert that aneurism never occurs without preceding degenerative changes in the arterial wall. Naturally, strain is the physiological burden of the aorta, and this strain tends sooner or later to degeneration of the arterial tunics. Then, given a weakened spot, the ordinary occurrences of every-day life are sufficient to precipitate disaster. A sneeze, a cough, some sudden exertion of the body in lifting or moving, have been the starting-points of aneurism. All accumulated testimony indicates that sudden strain is more dangerous than prolonged uniform strain, and therefore some occupations are more productive of aneurism than others. Inasmuch as age, sex, occupation, and personal habits influence the development and nutrition of the aorta, it is obvious that they must exert an important influence upon the occurrence of aneurism.

All records agree that aneurism is pre-eminently a casualty of middle life, and a glance at the accompanying table, which I have prepared from an analysis of 69 reported cases, shows that the disease is most common between thirty and fifty years of age:

From 20 to 30 years of age, 4 cases.
" 30 " 40 " " 21 "
" 40 " 50 " " 29 "
" 50 " 60 " " 14 "
" 60 " 70 " " 1 case.
Youngest case, 20 years of age.
Oldest case, 72 years of age.

Crisp analyzed 551 cases, and reports 398 between the ages of thirty and fifty.

Beneke has found in his records of arterial measurements that the pulmonary artery greatly exceeds the aorta in circumference up to the age of thirty. After that period the aorta begins to increase with relatively greater rapidity, until in the forties it exceeds the pulmonary artery in size, and it maintains its superiority from that time forward. The aorta continues to increase in circumference throughout life, but after the age of fifty this increase is considered a senile dilatation rather than an actual growth. It is interesting to note that the era of greatest liability to aneurism coincides with that of most rapid aortic development.

Sex furnishes a distinction in the frequency of aneurism. In 82 cases I found that only a seventh were females; Crisp registers less than an eighth. The radically different occupations and habits of women may contribute somewhat to their relative immunity from aneurism, and their physiological development also seems in their favor. Beneke states that the blood-pressure during childhood is about the same for both sexes. From puberty onward it is greater in the male. This is due to the fact that after puberty the volume of the heart relative to the length of the body is less in the female than in the male, and at the same time the main arteries of the body relative to the length of the body are only a trifle narrower in women than in men. The pulmonary artery, indeed, is relatively a trifle wider in women than in men. It follows from this that the blood-tension in both the large vessels emerging from the heart is less in the female than in the male.

In general terms, it may be said that those people who are exposed to heavy labor, as mechanics, laborers, soldiers, porters, cabmen, etc., are more liable to aneurism than those who are less exposed to such straining efforts. Fixture of the chest during effort brings greater strain upon the heart and aorta, and therefore men who wield heavy hammers and sledges are especially liable to aortic disease. Constriction of the neck or forcible extension of the same during exertion is dangerous, because it thus happens that the arteries are stretched in their long diameter at the same instant that the blood-wave is expanding them laterally, and they are thereby subjected to double strain. I knew of a trotting horse which was killed by this very combination of strain upon the aorta. At the end of a trial of speed the animal refused to stop; whereupon a groom sprang forward, seized him by the bit, and threw his head strongly upward and backward. His carotids and aorta were thus stretched to full length at the moment when his heart was acting with great force. The horse dropped dead, and the autopsy revealed a rupture of the aorta.

The frequency of aneurism among the soldiers of the English army was long the subject of anxiety and thought to English surgeons. Finally, some bright man recognized one cause in the dress of the soldiers. They were obliged to wear a high stock, which constricted the neck and kept it stretched, and their trappings were adjusted so as to keep the body in a stiff and unnatural position. These objectionable details of the dress have been removed, and it is now claimed that aneurism is much less common in the army.

Syphilis and gout undoubtedly contribute to the formation of aneurism, because they both dispose to degenerative processes in the arterial tunics. Some writers, however, have laid too much stress upon syphilis. It was claimed that this disease was the cause of the great frequency of aneurism in the English army. Barwell, however, calls attention to the fact that aneurism has been 13½ times more frequent per 1000 men in the army than in the navy, and yet no one maintains that syphilis is more common in the army than in the navy.