118 PRINCIPLES OF PHYSIOLOGY 



from the tissues to the lungs, there to be 

 eliminated. 



61. In order that the blood may be 

 brought into close proximity to the tissues, 

 we find a system of tubes, the organs of the 

 circulation, known as arteries, capillaries, and 

 veins, and at one point of the circulation, 

 where the arteries begin and the veins ter- 

 minate, we find a contractile force-pump, the 

 heart The walls of the arteries near the heart 

 are thick, strong, and highly elastic ; in those 

 farther away we find the elastic wall gradually 

 becoming thinner, and a contractile wall of 

 non-striated muscle appears, and becomes 

 thicker as we pass onwards, until in the smaller 

 arteries, or arterioles, the muscular coat is 

 the most pronounced. The arteries terminate 

 in the capillaries, which form a network of 

 minute tubes, many having a diameter of not 

 more than the three-thousandth of an inch. 

 These capillary networks bring the blood 

 close to the living tissue elements. Some 

 tissues, and always those in which there is 

 great physiological activity, are more vascular 

 than others. The capillaries terminate in the 



