[Illustration: Fig. 9.--Schematic representation of the increased strain brought to bear upon the cells of the intima, Int., when the media, Med., undergoes a localized expansion through relative weakness. (After Adami.)] The essential lesion in arteriosclerosis of the aorta and large arteries is a degeneration in the middle coat. This may be brought about by a variety of poisons circulating in the body. In syphilis, for example, the initial lesion has been shown to be a mesaortitis. The media seems to be dissolved, the artery is consequently thinned, there is actual depression along the level of the vessel. The elastic fibers disappear and small-celled infiltration takes its place. The intima hypertrophies, layer upon layer being added in an attempt to restore the strength of the vessel. There is also, as a rule, rather pronounced hypertrophy of the adventitia. =Experimental Arteriosclerosis= Within the past few years many workers have attempted by various means, to produce arterial lesions in animals, chiefly rabbits and dogs. The present status is somewhat chaotic, some affirming and some denying that arterial changes follow the various methods employed. Following the injection of small, repeated doses of adrenalin over a certain period of time, changes occur in the arteries of rabbits which are arteriosclerotic in type, the essential lesion being a degeneration of the muscular and elastic tissue of the media with the consequent production of aneurysm in the vessel. This is said by some to be quite like the type of arteriosclerosis in man which has been so well described by Moenckeberg. The degenerations in the arteries following the experimental lesions are of the nature of a fatty metamorphosis, and later proceed to calcification. Barium chloride, digitalin, physostigmin, nicotin and other substances, as well as adrenalin, have been found to exert a selective toxic action on the muscle cells of the middle coat of the aorta. The infundibular portion of the pituitary body, the portion which is developed from the infundibulum of the brain, possesses an internal secretion, which, injected intravenously, causes a marked rise of blood pressure and slowing of the heart beat. So far as I know, this active principle of the gland has not been used in an attempt to produce experimentally the lesions of arteriosclerosis.