DISEASES OF THE BLOOD VESSELS 73 
aorta; 26 or 76 per cent, of the 35 cases had this distri- 
bution alone. The arch seemed never to he 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. 
There have been also in mammals five cases of mesial 
change which have given rise to the picture described by 
Monckeberg 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 associa- 
tion 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 degener- 
ated 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 degen- 
eration 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. 
In the Aves the distribution and anatomy of this 
process present some differences. The aorta is as usual 
most conspicuously the seat of change, but it is note- 
worthy that the dilatation or ampulla immediately above 
6 
