2Q2 CIRCULATORY APPARATUS. 



words, to a reactive overgrowtli. The aneurism eomes to be 

 invested on all sides by a close web of tough connective tissue, which 

 supplies whatever deficiencies there may be in the inner coats, 

 and serves as a bond of union between the outer surface of the 

 aneurism and the neics;hbourin<x oro;ans. 



The implication of the adventitia in the morbid process is 

 therefore wholly salutary. Its resources however are not inex- 

 haustible. First, it cannot, whatever may be its thickness, arrest 

 the growth of the anem^ism. It yields uniformly and at all 

 points to the centrifugal pressure of the blood, and can never 

 offer anything like the resistance which is opposed by the com- 

 bined strength of a healthy internal and middle coat. Some 

 notion of the demands made upon the resisting power of the 

 aneurismal walls may be got l)y observing the conflict of an 

 aneurism with the osseous system. Should an aneurism of the 

 ascending aorta press forwards against the sternum, or one of 

 the thoracic aorta against the spine, it begins by destroying 

 whatever structures may intervene between the blood and the 

 bone. Intima, media and adventitia, mediastinal connective 

 tissue and periosteum, disappear without leaving any trace, 

 under the stress of the contending forces. Next, the bone 

 begins to yield ; it yields slowly — so that denuded remnants of 

 it are often found projecting far into the sac — but still it yields 

 and is absorbed. The bone-tissue under these circumstances 

 passes through a peculiar fibroid metamorphosis (fig. 11) which 

 has been more fully described in § 41. This however can only 

 be regarded as a passing phase in its disintegration and liquefac- 

 tion. The result is a loss of substance in the bone. Aneurisms 

 of the ascending aorta often annihilate the whole of the manu- 

 brium, the upper part of the body of the sternum, and the 

 sternal end of the clavicle. Aneurisms of the descending aorta 

 excavate cup-shaped hollows in the bodies of one or more of the 

 vertebrae. 



Another hindrance to the conservative powers of the ad- 

 ventitia arises from the circumstance that these powers, which 

 depend in great measure upon the transfer of hyperplastic 

 activity to the neighbouring connective tissue, have the extent 

 of their operation limited by the extent of distribution of that 

 connective tissue. Every free surface which the aneurism 

 approaches may be fatal to the patient, inasmuch as it gives the 



