DESCRIPTION OF THE ARTERIES 191 



of blood through the vascular system. Owing to the 

 small caliber of the arterioles the blood meets with 

 considerable resistance in passing through the arterioles. 

 As a result, there is a marked decrease in the pressure 

 in the arterioles and capillaries, due to this great 

 resistance, which is called the peripheral resistance. 

 The latter is caused by the small diameter of the 

 vessels modified by the tonic contraction of the 

 muscles in the wall of the arterioles. 



A practical idea of the blood-pressure can be obtained 

 from observing a cut or injured bloodvessel. If a 

 large artery, the blood will be seen to project from 

 the cut end nearest the heart, as a bright red fluid 

 spurting from the vessel, with considerable force, 

 dependent on the degree of pressure which it had 

 been subjected to in the vessel, and the tension of 

 the vessel wall before the injury. A vein when injured 

 bleeds with no spurting or force. It is seen as a dark 

 blue fluid coming from the cut end away from the 

 heart, as a steady stream (welling up). These differ- 

 ences in the characters of the hemorrhage from an arte- 

 rial and venous course indicate the difference of blood- 

 pressure between the arterial and venous systems. 



The venous pressure continues to fall from the 

 capillaries to the heart. There is simply a steady 

 tone to the walls of the veins which propel the blood 

 to the right side of the heart without any pulsation. 



The capillary pressure is dependent on the blood- 

 pressure of the arterioles and venous systems. It 

 is too minute to observe under normal physiological 

 conditions. 



THE DESCRIPTION OF THE ARTERIES, THEIR 

 DISTRIBUTION, ETC. 



There are two great arterial systems: (1) The 

 pulmonary, to the lungs; (2) the corporeal, to every 

 other part of the body. 



