DILATATION OP AKTERIES. — xVNEUKISM. 2G1 



pliagus or the trachea from the front. Aneurisms of tlie thoracic 

 aorta spring by preference from the back and sides of the vessel 

 and press against the spine. Aneurisms of smaller arteries are 

 much less frequent ; among these the most common are aneu- 

 risms of the popliteal, carotid and basilar arteries. 



§ 226. The most interesting question for the histologist is 

 that concerning the behaviour of the various structural com- 

 ponents of the arterial tube during its gradual dilatation— the 

 beliaviour of tlie inner, middle and outer coats respectivelv. 



Attention has already been drawn to the fact that tlie 

 intima is usually thickened by previous endoarteritis. This 

 enables it to take part in the dilatation without becoming notablv 

 thinner. Aneurisms from fjur to six inches in diameter mav be 

 lined with an internal coat of normal thickness ; nav, of more 

 than Jiormal thickness in parts. It exhibits the most diverse 

 stages of atheromatous degeneration side by side ; foremost 

 among them however stands calcification ; calcified plates have 

 been found involving one-half or even the whole of the circum- 

 ference of the aneurism, converting it into a bony drum. More 

 common is a tesselated mosaic of smaller plates, which are verj- 

 liable to become detached at their edges, and so to give the first 

 impulse to the formation of coagula. Fatty degeneration, tliough 

 less widely distributed, is nevertheless of the utmost moment. 

 For it is this — particularly when it results in the production of 

 atheromatous ulcers — which ultimately leads to perforation of 

 the intima at some point or other, and so exposes the integrity 

 of the sac to the most serious risks. 



From the first outset of dilatation the middle coot remains 

 entirely passive. The transverse bundles of smooth muscular 

 fibres separate from one another and allow ever-widening fissures 

 to gape between them. They finally succuml) to fatty degenera- 

 tion. Even when an aortic aneurism is no bigger than the 

 closed fist, it is a difficult matter to find any vestige or remnant 

 of the middle coat in its walls. 



On the other hand the adventitia may be said, in the strictest 

 sense of the word, to " guarantee the sac against rupture." The 

 pressure of the growing aneurism acts upon this membrane in 

 just the same way as a gradually increasing or oft-recurring 

 pressure acts uj^on a stratum of unformed connective tissue else- 

 where ; it stimulates it to a chronic inflammatory, or in other 



