THE CIRCULATORY SYSTEM. 459 



The transverse portion of the arch may be dilated in a fusiform shape, 

 or there may be sacculated aneurisms at any point in its wall. The sac- 

 culated aneurisms usually reach a considerable size. They press 011 the 

 sternum and ribs in front, or on the oasophagus, trachea, and bronchi be- 

 behind. The large arteries given off from the arch may be occluded. 

 They cause death by pressure on the air passages, the oesophagus, and 

 the vena cava ; or may rupture externally or into the oesophagus, trachea, 

 bronchi, pulmonary artery, or pleural cavities. 



On the abdominal aorta aneurisms are usually sacculated. If they 

 are situated high up they may project into the pleural cavities; if 

 lower down, into the abdomen. They may compress and displace the 

 viscera, vessels, and nerves, and erode the vertebra?. They may rupture 

 behind the peritoneum, into the peritoneal cavity, the pleural cavities, 

 the inferior vena cava, the bronchi, the lungs, the duodenum, the colon, 

 the pelves of the kidney, or the posterior mediastinum. Eupture of the 

 aorta with the development of a long dissecting aneurism parallel to the 

 vessel may give rise to a condition simulating a double aorta. 1 



The coronary arteries may be dilated throughout, or may be the seat 

 of small sacculated aneurisms. 11 These may rupture into the pericar- 

 dium, or may lead to rupture of the heart wall. 



The pulmonary arteries are rarely the seat of aneurisms. Diffuse and 

 circumscribed dilatations, however, sometimes occur on the main trunk 

 and on the two principal branches of the artery. They do not usually 

 reach a large size, but may cause death by rupture. General dilatation 

 of all the branches of the pulmonary artery is more common. It is 

 found in connection with stenosis of the mitral valves and with compres- 

 sion or induration of the lung tissue. Small multiple aneurisms miliary 

 aneurisms may occur in the cerebral arteries; less frequently in the 

 pulmonary and meseuteric. These are probably due to some congenital 

 weakness of the walls at the point of formation of the aneurism. 



Of the other arteries of the body there is hardly any one which may 

 not become the seat of an aneurism.' 



STENOSIS AND OBLITERATION OF THE AORTA. 



This lesion near the entrance of the ductus arteriosus has been observed 

 in a considerable number of cases. The degree of stenosis varies. The 

 aorta may be entirely closed and converted into a solid cord for a half an 

 inch ; or there may be a constriction through which there is a larger or 

 smaller opening. The walls of the aorta at this point may be thickened 

 and sclerosed. The ductus arteriosus may be closed or open. Above 

 the constriction the aorta is usually dilated ; below it, it is normal, dilated, 

 or steuosed. 



1 See case reported by G. P. Biggs, Trans. New York Path. Soc., 1897-98, p. 109. 



2 Consult Capp, Am. Jour. Med. Sciences, vol. cxviii., p. 812, bibliography; also 

 Griffith, Brit. Med. Jour., February 3d, 1901, p. 266, bibliography. 



3 For details concerning form, distribution, and frequency "of aneurisms, see the 

 larger works on the practice of medicine or surgery. 



