VASCULAR SYSTEM. 115 



the foetus. All the other canals approximate their inner- 

 surface, and are closed when their distending fluids or 

 contents are taken away. 



Q. How does a flattened, pressed, or bent artery reco- 

 ver its calibre and form ? 



#. By its elasticity. 



Q. What do you mean by the locomotion of an artery ? 



#. It is the rising of an artery, and the straitening of 

 its tube, particularly at the curves, when penetrated by 

 blood or injection, and its return to the natural state after- 

 wards. This is principally owing to the elasticity of the 

 arteries; veins manifest no such properties. 



Q. On what does this arterial locomotion depend? 



#. To it three circumstances are essential ; an impel- 

 ling power as that of the heart, the tortuosities in the ves- 

 sels to be strained by the blood impelled, and firm elastic 

 coats. 



Q. Can you distinguish between arterial elasticity and 

 contractility ? 



A. Elasticity is a physical property, contractility an 

 inherent textural one : elasticity presupposes compression ; 

 contractility presupposes the absence of distending con- 

 tents: elasticity is quick and sudden in its commencement 

 and termination ; contractility is slow with a permanent 

 tendency to contract: contractility presumes extensibility; 

 elasticity does not. 



2. Properties of Texture. 



Q. How do the arteries manifest extensibility ? 

 #. In two ways; in their diameter and in their length, 

 The longitudinal extensibility is greatest. 



