92 THE GENERAL CIRCULATION. 



stretched it reached to ten inches and a half, and when let 

 alone it contracted to six inches, when it became stationary; 

 the difference between six and ten and a half inches, is 

 the measure of its elastic force, while half an inch is due 

 to muscular contraction. 



Mr. Arnott makes the following statement, proving the 

 contractility of the arteries. 1. A small living artery cut 

 across, soon contracts so as to close its canal and arrest 

 hemorrhage; 2. While an animal is bleeding to death, the 

 arteries, accommodating themselves to the decreasing quan- 

 tity of blood, contract far beyond the degree to which their 

 simple elasticity would carry them. 3. The artery of a living 

 animal, if exposed by dissection to the air, sometimes will 

 contract in a few minutes to a great degree, and in such a 

 case only a single fibre of the artery may be affected, 

 narrowing the channel like a thread tied round it. 4. 

 When a living artery is tied, the part between the ligature 

 and the nearest branch on the side of the heart, gradually 

 contracts and becomes at last a solid and impervious cord. 

 The property of contractility in the arteries is admitted 

 by all, but that it is due to muscular structure is the point 

 in dispute. One point of distinction, as mentioned between 

 the contraction of arteries and that of muscle, is that the 

 former cannot be excited under the strongest electric and 

 galvanic stimuli, while the muscles can. 



Elasticity has been stated to be another property of the 

 middle coat, by which, if the artery be contracted too much, 

 it will dilate, and if dilated too much it will again return 

 to its natural size arid upon this property, combined with 

 the muscular contractions of the heart, depends the jetting 

 of the blood observed in the arteries. 



The internal coat of an artery is smooth, resembling se- 

 rous membrane, and a continuation of that lining the cavity 

 of the heart, which by its duplication forms the valve found 

 at the mouth of the aorta and pulmonary artery. This 

 coat is connected to the middle by fine cellular tissue, and 

 in advanced age is often subject to ossification or calcareous 

 concretion. 



