Most Forms Of Dilatation commonly included under Aneurism (spontaneous aneurism), and at the same time the most important of all, are owing, as we have already shown (p. 199), to the deposition of a tissue, analogous to the lining membrane of the vessel, derived from the blood, and occurring upon the inner surface of the artery, and to consecutive disease of the circular fibrous coat and the cellular sheath. Attempts based on an examination of the mode of construction of their walls, have been made to separate these dilatations into true and false or spurious aneurisms; or in accordance with a principle at variance with Scarpa's view of the cellular sheath of the vessel, into true and mixed aneurism, the latter being subdivided into external mixed and internal mixed or hernial aneurism. This class, moreover, comprises the dilatations which Cruveilhier and others, distrustful of the results of anatomical examination, divided merely in accordance with their external forms, into diffuse, fusiform, cylindroid and sacciform aneurisms. The last class were divided by Cruveilhier into "A. sous V aspect d'ampoules" with subdivisions into "A. péripheriques, semiperipheriques," and "à bosselures," and "A. sous l'aspect de poches a collet".

These dilatations considered under the common term of aneurism - spontaneous aneurism - will form the subject of the following remarks.

We must, however, begin by observing that a classification of aneurisms, simply based on the anatomical conditions of the coats of the artery, can only have reference to gradual disturbances, and cannot, therefore, afford a representation of essentially different and well-defined species, where the grounds that give rise to the formation of aneurism are the same. Although, indeed, a division derived from external form may have some practical utility, it cannot afford a sound classification, inasmuch as it has no reference to the anatomical disturbance on which the form depends, nor can it separate well-defined species, owing to the numerous transitions of form which they undergo. - We do not purpose giving any special description of the arrangements made by different ob-servers of these forms, since the references made to them in the appropriate parts of our work will be sufficient for their correct apprehension.

Spontaneous Aneurism appears in its simplest type as a diffused dilatation of a vessel towards all points of its surface in a cylindrical form, or when, as is usual, it gradually decreases towards both its extremities, till they assume the normal calibre, it is fusiform (the A. diffusum cylin-droideum of some writers, Cruveilhier's A. sous l'aspect d'ampoules périphériques).

Where the diseased condition of the coats of the vessel affects a more or less sharply-defined and considerable portion of the surface, the vessel dilates at that point in the form of an originally shallow pouch, which is gradually converted into a sac, flattened at its edges, where it is in contact with the interior of the artery, constituting saccular Aneurism, Cruveilhier's A. sous l'aspect d' ampoules semiperipheriques. The same is the case when, in diffused disease of the vessel, the morbid condition preponderates at any one point of the surface; the vessel being then generally dilated, but more especially at this point, whereas in the former case it is dilated into the form of a sac with ill-defined margins, and flattened at the spot where it branches off from the vessel.

It will be readily understood that a cylindrical or fusiform aneurism may very commonly be converted into a saccular aneurism, in consequence of the preponderance of disease at one or other point of its walls; or, even in the absence of disease, this change of form may arise at any point especially exposed to the force of the blood-wave, as for instance at the convexity of the ascending aorta.

These aneurisms are not only remarkable for their frequency, but more especially for the extraordinary size which they very commonly attain. The most important among them are those in which the saccular expansion affecting a portion of the periphery of the vessel is so situated that the wall opposite to the sac retains its normal form and direction. These approach to the pedicled aneurisms.