The Blood and its Circulation 



smallest veins. They are distributed through every tissue 

 of the body, except the epidermis and its appendages, the 

 epithelium, the cartilages, and the enamel of the teeth. 

 / 224. The Work done by the Capillaries. The capillaries 

 are closed vessels, and the space between the walls of the 

 capillaries and the cells of the tissues is filled with lymph. 

 As the blood flows along the capillaries, certain parts of 

 the plasma of the blood filter through their walls into the 

 lymph, and certain parts of the 

 lymph filter in the opposite direc- 

 tion through the walls of the capil- 

 laries and mingle with the blood 

 current. 



A similar exchange of material 

 is constantly going on between the 

 lymph and the tissues themselves, 

 so that the lymph acts as a medium 

 of exchange between the blood and 

 the tissues. 



In brief, the tissues, with the lymph as a kind of "middle- 

 man," may be said to live on the blood. 



.Experiment 63. Grasp the left wrist tightly with the thumb and 

 two fingers of the right hand. Note the little knots or swellings in 

 the veins caused by checking the flow of the venous blood towards the 

 heart. These swellings show the location of the valves of the veins. 



225. The Pulmonary, or Lesser, Circulation. Let us now 

 study the circulation as a whole, tracing the course of the 

 blood from a certain point until it returns to the same 

 point. We may conveniently begin with the portion of 

 blood contained at any moment in the right auricle. 



The superior and inferior venae cavae are busily filling the 

 right auricle with dark, impure blood. When it is full it 

 contracts. The passage leading to the right ventricle lies 



FIG. 75. The Structure 

 of Capillaries. 



(Capillaries of various sizes, 

 showing cells with nuclei.) 



