Phlebosclerosis not infrequently occurs with arteriosclerosis. It is seen in those cases characterized by high blood pressure. Such increased pressure in the veins is due, for example, to cirrhosis of the liver which affects the portal circulation, or to mitral stenosis which affects the pulmonary veins. The affected vessels are usually dilated. The intima shows compensatory thickening especially where the media is thinned. As a rule all the coats are involved in the connective tissue thickening. Occasionally hyaline degeneration or calcification of the new-formed tissue is seen. "Without existing arteriosclerosis the peripheral veins may be sclerotic usually in conditions of debility, but not infrequently in young persons." (Osler.) In many cases of arteriosclerosis, the pathologic changes are not confined to the arteries, but are found in the veins as well as in the capillaries. Such cases could be called angiosclerosis. CHAPTER III PHYSIOLOGY OF THE CIRCULATION No attempt will be made to cover the entire subject of the physiology of the circulation. Only in so far as it relates to arteriosclerosis and blood pressure and has a bearing on the probable explanation of blood pressure phenomena will it be discussed. "The heart and the blood vessels form a closed vascular system, containing a certain amount of blood. This blood is kept in endless circulation mainly by the force of the muscular contractions of the heart; but the bed through which it flows varies greatly in width at different parts of the circuit, and the resistance offered to the moving blood is very much greater in the capillaries than in the large vessels. It follows, from the irregularities in size of the channels through which it flows, that the blood stream is not uniform in character throughout the entire circuit--indeed, just the opposite is true. From point to point in the branching system of vessels the blood varies in regard to its velocity, its head of pressure, etc. These variations are connected in part with the fixed structure of the system and in part are dependent upon the changing properties of the living matter of which the system is composed." (W. H. Howell.)