20 



through the lungs, into the left ventricle. The for- 

 mer conveys it from the left to all other parts of the 

 body. 



Thearteries ordinarily consist of three coats or mem- 

 branes. The outermost has been generally thought 

 to be composed of fine blood-vessels. The second i^ 

 muscular, and made of firm and strong circular, or 

 rather spiral fibres: of which there are more or fewer 

 strata, as the artery is larger or smaller. These fibres 

 are extremely elastic. The inmost coat is a fine dense, 

 transparent membrane, containing the blood, which 

 otherwise would easily ooze through the spiral fibre. 



On a more accurate examination it has been found, 

 that the outermost coat of all arteries is a cellular sub- 

 stance, composed of fine, pellucid membranes, which 

 may be stretched, even suddenly to a great extent 

 without breaking. And they as suddenly collapse, 

 when that stretching force is removed. These cells 

 contain an oily liquor which their coats secern from 

 the branches of the artery that are spread over them, 

 Thiscellularsubstance of the arteries serves to connect 

 them with- the surrounding part, without hindering, 

 their actions or motions. It gives a safe passage to 

 the vessels of their oilier coats, and supplies oil for lu- 

 bricating them. There is also another cellular sub- 

 stance, between the membranes and the muscular 

 eoat. 



All the arteries begin with a larger trunk, and 

 grow less and less till they are no longer seen Dy the 

 naked eye. Hence they are continued till they inos- 

 culate with the veins, and so form one uninterrupted 

 channel. 



They appear white, because their coats are of so 

 dense a contexture, that the blood is not visible 

 through them.. This proceeding from wider to nar- 

 rower canals is continually obstructed in its passage, 

 but being pushed on from behind, it distends the coats, 

 and causes that leaping motion called the pulse. By 

 this, as well as by their whiteness, arteries are dis- 

 tinguished from veins. 



