Organs of Respiration and Circulation. 



58 [ 



Fii?. 5. 



A. An artery partly cut open to show its inner coat. 



B. A vein also opened, and showing c, its valves. 



in assisting- the circulation is a vexed question among physiologists, 

 but no doubt it is very considerable. The simple fact of these 

 vessels being found empty after death, goes very far to prove their 

 importance as active auxiliaries to the heart; for were they passive 

 tubes merely, they would then be in the opposite condition, viz., 

 full. The early anatomists were acquainted, with the circum- 

 stance of these vessels being void of blood after death, and conse- 

 quently they were led to suppose that the ' animal spirits, being 

 of an aeriform nature/ were conveyed by them ; hence the name 

 artery, or air-carrying tube. At their origin these vessels are large, 

 but they gradually decrease in size as they proceed from the heart, 

 which is produced in part by the number of the branches they give 

 off in their course. It has been rightly said that the capacity of 

 the arterial system is rather increased than diminished by this sub- 

 division, and therefore no mechanical obstructicm from that cause 

 can interfere with the flow of the blood in the small arteries. 

 The rapid splitting up of these vessels into smaller ones is in 

 VOL. X. 2 Q 



