ARTERIES AND VEINS. 317 



anatomists, as similar to the serous membranes, though I am not 

 aware that any experiments have been made to determine the 

 point 



The middle coat is the principal one, and the one to which the 

 arteries are chiefly indebted for their peculiar characters. It 

 consists of pale, straw-coloured fibres, coiled obliquely round the 

 circumference of the vessels, but none of them forming a com- 

 plete circle. If an artery be stretched transversely it will recoil 

 and resume its original diameter. If elongated it will retract. 

 We see from this that arteries are highly elastic, and this pro- 

 perty they owe chiefly to the middle coat, which is strong and 

 dense. When an artery no longer carries blood, as after a li- 

 gature has been applied to it, the part beyond the ligature will 

 retract, its cavity will be obliterated, and, by an alteration in its 

 mode of nutrition, will degenerate into a fibrous cord. This in- 

 dicates a contractile power differing from mere elasticity, and has 

 been termed contractility of tissue. Anatomists long ascribed 

 muscular properties to this middle arterial coat ; but the chemi- 

 cal properties which it possesses are incompatible with this no- 

 tion. 



The middle arterial coat is quite insoluble in water, even when 

 long boiled in that liquid. When concentrated acetic acid is 

 poured on it, it neither softens nor dissolves ; nor do we obtain 

 any solution even when we boil it in dilute acetic acid. But it 

 dissolves with great ease in sulphuric, nitric, and muriatic acids, 

 even when much diluted with water. The solution is neither 

 precipitated by an alkali nor by prussiate of potash, as is the 

 case with fibrin, and with muscular fibre treated in the same way. 



The middle arterial coat is dissolved by caustic potash. The 

 solution is colourless, but slightly muddy ; and it is not precipi- 

 tated by acids. If we mix together saturated solutions of the 

 middle coat of an artery in potash, and in an acid, the mixture 

 becomes gradually muddy, and a precipitate falls. 



This middle coat, after having been purified by solution in di- 

 lute potash ley and precipitation by an acid, was subjected to 

 analysis by Dr Scherer. * When burnt it left an ash weighing 

 1-7 per cent. Its constituents (abstracting the ash) were found 

 to be 



* Ann. der Pharm. xl. 51. 



