TEXTURE OF THE ARTERIES. 169 



of the heart begins to fail, and increasing in strength as that 

 decreases in power/'* 



*s 



The Internal Coat of the arteries is designated by the terms 



Nervous and Arachnoid. It is continued from the ventricles of 

 the heart, in the left one of which it is of unusual thickness. It 

 is the duplication of this membrane with some fibres interposed, 

 that composes the semi-lunar valves of the aorta and of the pul- 

 monary artery. Its internal face is smooth, polished, and moist- 

 ened with a kind of humidity which permits the blood to flow 

 through with diminished friction. In the larger arterial trunks, 

 some small longitudinal wrinkles are observable in it; and when 

 an artery has been cut through, as in amputation, it is disposed 

 to retract in small transverse wrinkles/ It is, therefore, not 

 very extensible, but has, "according to the experiments of Sir 

 Everard Home,t a considerable degree of solidity and strength. 

 Ossifications of this membrane are very frequent after the age 

 of sixty. 



In addition to the tunics mentioned, cellular substance, vessels, 

 and nerves enter into the structure of arteries. 



The Cellular Substance is not abundant, and serves principally 

 to unite the sides of the circular fibres to one another, and to 

 join the internal to the middle coat. 



The Vessels (J^asa Arteriarum) consist both. in arteries and in 

 veins, and come from the adjacent trunks, instead of from those 

 on which they ramify. They may be made very distinct by a 

 fine injection, or by laying them bare in the living body; when 

 in a little time after exposure, they begin evidently to carry red 

 blood, and to grow turgid as in inflammation. The difference 

 in the colour of the blood distinguishes these arteries from the 

 same kind of veins. Both arteries and veins may be traced very 

 well into the middle coat, but not upon the internal, though the 

 changes which occur in the latter, from disease and upon the 

 application of ligatures, prove clearly that exhalation and ab- 

 sorption are continually going on there. For in inflamed arte- 

 ries, an exhalation is seen upon their internal surface, and when 



* Hunter, loc. cit. 



f Transactions for the Improvement of Medical and Surgical Knowledge 

 vol. i. 



VOL. II. 16 



