The group to which the name degenerative arteritis has been applied is, as has already been indicated, more productive than the analogues seen in the human being, but indeed it is questionable whether the lesions even in the lower animals are not more degenerative than productive. Since, however, chronic arteritis is always associated with damage to the elastic and muscular fibres of the media as well as with fatty change and overgrowth of the intima, all the deforming and degenerative cases will be classed together. The general picture in mammals is one of diffuse rather than of plaque- like thickening, but well outlined raised or depressed areas are encountered. In the aorta and larger branches one may find irregular streaking and loss of elasticity with fairly clear, pale yellow or gray, flat sections of distinct opacity. Rarely these may contain calcareous matter, a change most often seen in the carnivores. The lesions are very largely limited to the aorta; 26 or 76 per cent. of the 35 cases had this distribution alone. The arch seemed never to be affected alone, and indeed it is rather commoner to find opaque patches stretching along the thoracic or even abdominal portion; this is especially true of the Ungulata. [Illustration: FIG. 2.—ARTERIOSCLEROSIS AND ATHEROMA. THORACIC AORTA. JACKAL (CANIS AUREUS). THIS WAS CONTINUED TO THE MESENTERIC AND ILIAC VESSELS. ] There have been also in mammals five cases of mesial change which have given rise to the picture described by Mönckeberg and usually entitled by his name. However, the noteworthy differences between the wild animal and the human cases are the absence of advanced calcification in the media under the concavities and the prominence of the changes in the aorta near the heart to be found in the former. These few cases do not permit an association of the arterial disease with any particular pathology in other parts. Considered minutely, the outstanding lesion in the class Mammalia is the separation of the elastic fibres by fluid and debris, apparently derived from the degenerated muscle fibres, associated with a decrease of round and elliptical nuclei. Globules and hyaline pink staining material are often collected between split-up elastic strands, which fibres in some cases seem quite numerous, in others reduced. In the intima heaping-up of cells and fibres is very moderate in degree while usually one finds only subendothelial edema. When the process has advanced far, the microscopy is like that of well developed human lesions. Arterial degeneration due to parasites gives a different picture in that medial degeneration is far advanced and some fibrinocellular activity is seen upon the intima when this tissue remains. When, however, the infestation has proceeded to weaken the wall sufficient for it to give way into an aneurysm, little or no vestige of the true arterial wall is left.