460 THE CIRCULATORY SYSTEM. 



Stenosis of the aorta is followed by hypertrophy of the left ventricle, 

 and, later, of the right ventricle, with venous congestion throughout the 

 body; or there may be a collateral circulation developed between the 

 arteries given off above and below the constriction ; or there may be rup- 

 ture of the aorta, the right ventricle or auricle. 



This condition is found at all ages, but is induced during foetal life 

 or in the first year of extra-uterine life. It is probable that it may be 

 caused after birth by an abnormal closure of the ductus arteriosus. This 

 vessel normally becomes closed without the formation of a thrombus. 

 If a thrombus be formed it may extend into the aorta and obstruct it ; or 

 the ductus arteriosus is filled with a thrombus, but increases for a time 

 in size ; afterward, as the thrombus is absorbed, the vessel contracts and 

 draws the walls of the aorta together. 



TUMORS OF THE ARTERIES, 



Secondary tumors, chiefly carcinomata and sarcomata, may occur in 

 the walls of the arteries by continuous growth from without, involving 

 first the external layers. To these layers they are usually confined, for 

 the density of the inner layers affords such marked resistance to the in- 

 filtration of the tumor cells that arteries are apt to pass intact through the 

 tumor, which grows around them. More frequently the arteries become 

 secondarily involved in the growth of malignant tumors by the occur- 

 rence within them of emboli formed by larger and smaller masses of 

 tumor cells. , 



These emboli are usually of small size, and are apt to get into the cir- 

 culation by growing through the walls of the veins into their lumiua. 

 Large emboli from tumors are most apt to occur in the branches of the 

 pulmonary artery. The emboli, formed as they are for the most part by 

 cells capable of growth and proliferation, are apt soon to form connec- 

 tion with the walls of the vessels, and, by the growth into them of blood- 

 vessels from the vasa vasorum to find the conditions necessary for their 

 development, and they may thus soon involve the entire wall of the ves- 

 sel and grow out into adjacent parts. 



The Veins. 

 DILATATION. (Phlebectasia.) 



Dilatation of the veins, or phlebectasia, presents itself under a vari- 

 ety of forms. 



1. SIMPLE DILATATION. The vein is uniformly dilated in a cylindri- 

 cal or fusiform shape ; its length is not increased ; its walls may be thicker 

 than normal or thinned ; the valves may be thickened, or are insufficient, 

 or atrophic, or are torn. 



2. CIRSOID DILATATION. The vein is cyliudrically dilated, but is 

 also increased in length, so that it assumes a very tortuous course. The 

 walls may be thickened or thinned. 



