However unfavorable the ordinary termination of an aneurism may be, instances are occasionally observed in which the disease takes a more favorable turn, and nature brings about a spontaneous cure of the aneurism. This result is effected in many different ways, which have been especially considered by Hodgson.

1. The aneurism may compress the artery on which it is seated in such a manner, either above or below, that it gradually becomes impervious, and is then obliterated with the aneurism. We attempt to imitate this healing process artificially, by passing a ligature either above or below the aneurism. Such a result can only affect saccular aneurisms, and such as are attached by a neck.

2. The aneurism may be completely filled with fibrinous coagula above which a deposit is formed, which represents the lining coat of the vessel and stops the communication between the cavity of the aneurism and the tube of the vessel. Aneurisms attached by means of a neck, and having only a narrow passage of communication, present the most favorable conditions for this mode of termination. We even observe, in some rare cases, that where saccular or spindle-shaped aneurisms have been completely filled with fibrinous coagula, new formations continue to be deposited upon the former, until at length the whole diseased vessel becomes obstructed. Decrease in the general quantity of the blood and a diminution of the heart's action must be regarded as the most favoring influences.

In both these conditions the aneurism shrivels and contracts over the coagula, either in the form of a fibroid capsule or of a spindle-shaped cylindrical roll, and is then atrophied.

3. In aneurisms in the extremities the gangrenous process to which they give rise attacks the aneurism itself, and by exciting arteritis, causes the artery to be stopped up by a coagulum. The aneurismal sac is thrown off and removed, and the artery obliterated at various parts. In the same manner, abscesses and inflammatory foci in the vicinity of an aneurism may occasion arteritis, accompanied by occlusion, and subsequent obliteration of the artery, by which the aneurismal sac is destroyed, and removed by suppuration.

Spontaneous aneurism generally occurs with the same proportional frequency in the two arterial systems, and in the different portions of the aortic system as the disease of the coats of the vessel on which they are based (see p. 204). The relative scales of frequency established by different observers are indeed tolerably accurate; but still many of the results which have been given are incorrect; thus, for instance, the assumption of the great frequency of aneurism of the popliteal artery, is undoubtedly so far incorrect that it includes in the same class with spontaneous aneurisms of the lower limbs, which are certainly not of rare occurrence, many others which very probably were of traumatic origin.

In general, aneurisms are incomparably more frequent in the larger than in the smaller arteries, and their occurrence on the trunk of the aorta is characterized by remarkable frequency.

Aneurisms are, on the contrary, very rarely observed in the pulmonary arterial system, where, as far as we know, they are limited to the trunk. Fusiform and saccular dilatations do certainly sometimes occur in the ramifications of the pulmonary artery, within the parenchyma of the lung, as we have observed near tuberculous caverns; but as they originate from entirely different causes, they do not belong to this class (compare p. 198).

Aneurism of the trunk of the pulmonary artery is scarcely ever present, unless there is at the same time aneurism of the aorta, or, at all events, a tendency to that disease.

Although it may be said in reference to the sex most frequently affected that there is a preponderance in men, it is by no means so considerable as is usually supposed. The age at which aneurisms are most common is between the 30th and 60th year; they are of much rarer occurrence between the 20th and 30th year, and must be regarded as extremely unfrequent, and as exceptional cases, when they are present before the age of 20 years. We must, however, exclude from this calculation all the aneurisms of traumatic origin which have hitherto not been separated with sufficient care.

It has long been supposed that aneurisms are based on a special aneu-rismal diathesis, in consequence of the frequency with which they have been observed to appear spontaneously and independently of external influences, and from the fact that several occur simultaneously or in quick succession to each other in the same individual. Thus incidental and individual cases have led to the idea that these predisposing conditions were to be sought in gout, syphilis, or mercurial cachexia; and this opinion was supposed to derive support from the more frequent occurrence of aneurisms in men, as well as from the period of life at which they are most commonly observed. It was conjectured that this diathesis gave origin to the diseased condition of the texture of the coats of the vessel, and to their loss of elasticity and their softening and brittleness.

We have sufficiently considered the anatomical bearings of the disease affecting the coat of the vessel on which the formation of spontaneous aneurism depends (see p. 199), and have drawn attention to an anomaly of the blood-crasis, which may give origin to an aneurism (see p. 206). Nothing positive is known in reference to this blood-crasis; but the concurrence of the above-named diseases with aneurism appears to us to be purely accidental, nor do we think that such individual cases afford sufficient scientific grounds for the connection that has been supposed to exist between these diseases and the aneurism.

It is a very important fact that spontaneous aneurism never exists in combination with tuberculosis. This immunity is based on the following grounds: a. The diseased condition of the coats of the vessel on which aneurisms depend, constitutes a cause of immunity against tuberculosis (see p. 206).

b. Large aneurisms of the aorta give rise to consecutive disease of the heart in the form of dilatation, with a readiness proportional to their vicinity to the heart. It is, therefore, in consequence of the venosity and cyanosis occasioned by the latter disease, that aneurisms of the aorta afford a decided immunity against tuberculosis.

Aneurisms, as we have already seen, have nothing in common with cancer.

It still remains for us specially to notice several particular forms of aneurism.