CAUSE OF THE PULSE 



173 



2 



Valves in a 

 vein. (Explain 

 from the text.) 



have small valves which open in the direction of the blood flow. In the figure 

 1 is a diagram of a section of a vein when the blood flows properly ; 2, when 

 the blood is checked (valve closed) ; 3, valves as they appear in 

 a vein cut open. Capillaries are a network of very thin-walled 

 vessels through which food, oxygen, and colorless corpuscles 

 pass out to the tissues. 



Cause of the Pulse. — The pulse, which can easily be de- 

 tected by pressing the large artery in the wrist or the small 

 one in front of and above the external ear, is caused by the 

 gushing of blood through the arteries after each pulsation of 

 the heart. As the large arteries pass away from the heart 

 and divide, the diameter of each individual artery becomes 

 smaller. At the very end of their course, these arteries are so 

 small as to be almost microscopic in size and are very nu- 

 merous. There are so many that if they were placed together, 

 side by side, their united diameter would be much greater 

 than the diameter of the large artery {aorta) which passes 

 blood from the left side of the heart. This fact is of very great importance, 

 for the force of the blood as it gushes through the arteries becomes very much 



less when it reaches the smaller ves^ 

 sels. This gushing movement is 

 quite lost when the capillaries are 

 reached, first, because there is so 

 much more space for the blood to 

 fill, and second, because there is con- 

 siderable friction caused by the very 

 tiny diameter of the capillaries. In 

 the veins the blood pressure is more 

 uniform and the movement of the 

 blood slower than in the arteries. 

 It is estimated that blood in the 

 large arteries travels about 16 inches 

 a second, in the capillaries from ^ 

 to ^ of an inch a second, and in the 

 large veins about 4 inches a second. 

 This shows us that there is a very 

 ninV ^^^B ^^ considerable pressure exerted by the 

 Inlv ^9^^ blood on the arterial walls at each 



^mPW ^ stroke of the heart. 



Lymph Vessels. — The lymph is 

 collected from the various tissues of 

 the body by means of a number of 

 very thin-walled tubes, which are at 

 first very tiny, but after repeated connection with other tubes ultimately unite 

 to form large ducts. These lymph ducts are provided, like the veins, with valves. 

 The pressure of the blood within the blood vessels continually forces more 



The lymph vessels. The enlargements 

 are lymph glands. The arrow indicates 

 where most of the lymph returns to the 

 circulatory system. 



