240 PHYSIOLOGY AT THE FARM. 



a reticular manner. The muscular tissue of the middle coat is 

 more pure in the smaller arteries. 



The external coat has two layers of different texture 

 namely, an internal stratum of genuine elastic tissue, most 

 obvious in arteries of large size, and at length disappearing in 

 small arteries ; and an outer layer, consisting of ordinary areolar 

 tissue, in which the filaments are closely interwoven, and in 

 large and middle-sized arteries, run diagonally or obliquely 

 round the vessel. The areolar layer is commonly of great pro- 

 portionate thickness in the smaller arteries. 



The vital contractility of the arteries is a subject which 

 has often engrossed the attention of physiologists. It has been' 

 frequently confounded with their elasticity. The contraction 

 of the artery that immediately follows its distension by the 

 wave of blood received from the heart, appears to be due 

 solely to the elasticity of the artery. The vital contractility, 

 on the other hand, produces its effect by a slow contraction of 

 such a kind as may accommodate the artery to a larger or 

 smaller volume of the contained blood. It is identical with 

 what was formerly described as tonicity. The importance of 

 such a property in the arteries can be readily conceived. If 

 this property should by any cause be diminished in any part 

 of the arterial system, that part will immediately appropriate 

 to itself more than its former share of blood ; if, on the other 

 hand, the contractile property become more energetic, then the 

 part of the arterial system so affected will contain less than 

 its usual share of blood. To this property is due the empti- 

 ness of the arteries after death, for as soon as the heart's 

 action ceases, or even as soon as it becomes much weakened, 

 the force which before tended to keep open the arteries fails, 

 and therefore the tonicity or slow contractile property begins 

 to expel the blood into the veins ; but as vital contractility 

 begins to fail soon after death, the elasticity of the arterial 



