A STUDY OF THE CIRCULATION OF BLOOD 137 



are so thick that none of it can soak out into the muscle 

 even if there were time to do so. Just beyond the semilunar 

 valves of the aorta, two arteries (the cor'o-na-ry arteries) 

 are given off ; these at once pass over and through the walls 

 of the right and left hearts (like a crown; hence, Latin 

 corona = crown), sending branches into every part of the 

 working tissue. 



2. THE BLOOD VESSELS 



Position of Arteries and the Pulse. We have defined an 

 artery as a blood vessel carrying blood from the heart. 



A V 



FIG. 51. Cross Section of an Artery (A), and of a Vein (7). 



c connective tissue. m = muscle layers. 



e.c = cells of serous lining. n = nuclei of serous cells. 



Every time the ventricles contract, we have seen that the 

 aorta and pulmonary artery are stretched ; this is true of 

 every artery in the body. Most arteries lie beneath thick 

 layers of muscle or bone, which protect them from possible 

 injury; but in certain regions of the body they run close to 

 the surface. If one places one's forefinger on the wrist of 

 one's other hand, just at base of the thumb, one can feel a dis- 

 tinct beating, called the pulse. This is due to the enlarge- 

 ment of the artery at each heart beat. When an artery is 

 cut, therefore, the blood is forced out in spurts at each con- 

 traction of the ventricle. 



Structure of Arteries. If one cuts off a piece of the aorta 

 of any animal, one finds that the blood vessel retains its 



