168 CIRCULATORY SYSTEM. 



amount of its elastic power, while only half an inch of contrac- 

 tion was due to the muscular stratum, or, in other words, an 

 eleventh of the whole. 



A section of the iliac artery, measuring two inches in circum- 

 ference, on being allowed to contract after stretching, measured 

 two and one-third inches; it, therefore, gained one-sixth the 

 amount of its muscular contraction. A section of the axillary 

 artery gained one-eighth of the carotid, two-thirds of the ra- 

 dial artery, doubled its primitive extent. From all which the 

 inference was drawn, that the power of recovery in a vessel is 

 greater, in proportion as it is nearer the heart, but lessens as the 

 distance increases, which shows the decrease of elastic, and the 

 increase of muscular power. 



The elastic coat gives a middle state to an artery, or has a 

 continued tendency to it; if, therefore, the artery be too much 

 dilated, it contracts it, and if it be too much contracted, it di- 

 lates it, all of which is readily exemplified by a cylinder of gum 

 elastic, which, whether compressed or dilated, has only one state 

 of repose, to which it immediately returns on being left to it- 

 self. Mr Hunter supposed, that a certain degree of elasticity 

 is continued to the very end of every artery, from this quality 

 being better suited to sustain a permanent resistance than mus- 

 cular power; as a pipe of lead, from its want of elasticity, 

 finally becomes stretched and useless under the pressure of a 

 column of water, whereas, one of iron, from being elastic, al- 

 ways re-acts efficiently. It is this elasticity in the arteries, 

 which causes the blood, at a little distance from the heart, to 

 flow through them in a continued jetting stream when they are 

 opened, although it is supplied to the aorta by interrupted strokes. 

 In this way, as the artery is more distant from the heart, the 

 stream bjcomes proportionately regular. 



"The muscular power of an artery renders a smaller force 

 of the heart sufficient for the purposes of circulation; for the 

 heart need only act with such force as to carry the blood through 

 the larger arteries, and then the muscular power of the arteries 

 takes it up, and, as it were, removes the load of blood while the 

 heart is dilating. In confirmation of this remark, it is observa- 

 ble in animals whose arteries are very muscular, that the heart 

 is proportionably weaker, so that the muscular power of the ves- 

 sels becomes a second part to the heart, acting where the power 



