GENERAL ANATOMY. 



417. Clifton Wintringham has examined the tenacity of 

 the arteries and the resistance they oppose to rupture. I have 

 also made some experiments on this subject. These vessels 

 have a great power of resistance, which is, generally, in pro- 

 portion to their thickness. That of the aorta is superior to 

 that of the pulmonary artery. In the same degree as the ar- 

 teries diminish in size, their absolute tenacity diminishes, but 

 their relative thickness and softness increase, their extensi- 

 bility and relative resistance augment. The resistance is not, 

 however, the same in all the arteries of the same volume: that 

 of the iliac artery is greater than that of the carotid. The te- 

 nacity in the longitudinal direction is almost entirely owing 

 to the outer membrane. The circular resistance, which is 

 much stronger, is owing to the middle and outer layers. The 

 inner membrane has very little power of resistance in either 

 direction. 



418. The most important physical property of the arte- 

 ries is their elasticity. If they be distended in the longitudi- 

 nal direction, they yield and elongate, to return suddenly to 

 their former state whenever the distention ceases. If they be 

 distended transversely, the}'' are less distensible and spring 

 back with greater force. If by injection or insufflation they 

 are distended to excess, they enlarge a little, elongate, and 

 when the effort ceases, they spring back upon themselves and 

 expel part of their contents. If they be bent, they return to 

 their former direction; if they be flattened by pressure, they 

 resume their cylindrical form. During life, they are in a 

 state of elastic tension, which, when they are divided, causes 

 the ends to retract. 



The largest arteries possess a very distinct elasticity, but it 

 diminishes successively in the smaller ones. 



419. The arteries are also susceptible of a slow extensibi- 

 lity and retractibility. When a principal artery is obliterated, 

 the collateral arteries, in replacing 'it in its functions, enlarge 

 and acquire, in a short time, a considerable volume; this en- 

 largement is of the same kind as ordinary growth, but is much 

 more rapid: the artery, on the contrary, which ceases to af- 



