It would seem that all the evidence now leads us to believe that the capillaries themselves are contractile and it is even possible that they may be under vasomotor control. If the anatomic structure as stated above, is correct, it would take but a slight contraction of the encircling cell to shut off completely the capillary. When the enormous capillary bed is considered, it is not inconceivable that circulating poisons may act on large areas and produce a true capillary resistance to the onflow of blood which might express itself, if long continued, in actual hypertrophy of the heart. CHAPTER II PATHOLOGY The whole subject of the pathology of arteriosclerosis has been much enriched by the study of the experimental lesions produced by various drugs and microorganisms upon the aortas of rabbits. Simple atheroma must not be confused with the lesions of arteriosclerosis. The small whitish or yellowish plaques so frequently seen on the aorta and its main branches, may occur at any age, and have seemingly no great significance. Such plaques may grow to the size of a dime or larger, and even become eroded. They represent fatty degeneration of the intima which, at times, has no demonstrable cause; at times follows in the course of various diseases, and undoubtedly is due to disturbances of nutrition in the intima. Except for the remote danger of clot formation on the uneven or eroded spot, these places are of no special significance, and are not to be confused with the atheroma of nodular sclerosis. The lesions of arteriosclerosis are of a different character. It has been customary to differentiate three types: (1) nodular; (2) diffuse; (3) senile. It must be understood that this is not a classification of distinct types. As a rule in advanced arteriosclerosis, lesions representing all types and all grades are found. The nodular type, however, may occur in the aorta alone, the branches remaining free. This is most often found in syphilitic sclerosis where the lesion is confined to the ascending portion of the arch of the aorta.