STRUCTURE OF ARTERIES. 143 



established by the results of several experiments, of which 

 the following is one of the most conclusive : Dr. Sharpey 

 injected bullock's blood into the thoracic aorta of a dog 

 recently killed, after tying the abdominal aorta above the 

 renal arteries, and found tha with a force just equal to 

 that by which the ventricle commonly impels the blood in 

 the dog, the blood which he injected into the aorta passed 

 in a free stream out of the trunk of the vena cava inferior. 

 It thus traversed both the systemic and hepatic capillaries ; 

 and when the aorta was not tied above the renals, blood 

 injected under the same pressure flowed freely through the 

 vessels of the lower extremities. A pressure equal to that 

 of one and a-half or two inches of mercury was, in the 

 same way, found sufficient to propel blood through the 

 vessels of the lungs. 



But although it is probably true that the heart's action 

 alone is sufficient to ensure the circulation, yet there is 

 reason to believe in the existence of several other forces 

 which are, as it were, supplementary to the action of the 

 heart, and assist it in maintaining the circulation. The 

 principal of these supplemental forces have been already 

 alluded to, and will now be more fully pointed out. 

 



THE ARTERIES. 



For the purpose of explaining the influence of the arteries 

 in the circulation it will be sufficient to consider the walls of 

 an artery as containing three principal coats : an external, a 

 middle, and an internal coat. The external coat is constructed 

 of ordinary areolar tissue, the fibres of which are chiefly of 

 the white, inelastic kind, especially towards the outer 

 portion of the artery. They are arranged, for the most 

 part, in a longitudinal or oblique direction. 



The following are the uses of the external coat : (i.) It 

 forms a strong, tough investment, which, though capable of 

 extension, appears principally designed to strengthen the 

 walls of the artery, and to guard against their excessive 



