62 



Physiology. 



the brain when we are studying, to the digestive organs after eating, etc. 

 The plain muscle fibers shorten at a much slower rate than the striated 



fibers. They are also slower in relaxing. 



Circulation of Blood in the Web of a 

 Frog's Foot. This is a beautiful sight. 

 Here you may see, under the microscope, 

 the active streams of blood. Small ar- 

 teries divide to form capillaries, and capil- 

 laries unite again to form the small veins. 

 In the narrow capillaries the corpuscles 

 may be seen moving along in single file, 

 with barely width enough to pass through 

 the slender tube. If you see this in the 

 frog's foot, you can understand how the 

 blood flows through all the active tissues 

 of your body. (See Figs. 37 and 39.) 



the Capillaries. The arteries 



Fig. 37. "Capillary Blood Tubes in 

 the Web of a Frog's Foot, under 

 a Low Power of a Microscope." 

 From Hall's " Physiology." 



The Blood Flow in 



divide and subdivide, and surface view 



become capillaries, which 

 have connecting branches, 

 forming a close network 

 of tiny thin-walled tubes. 

 These penetrate nearly 

 every tissue of the body. 

 The blood cannot do its full 

 work till it is in the tissues, 

 and to reach the tissues it 

 must soak through the walls 

 of the capillaries. The Fig. 38. 

 work of the heart and ar- 

 teries is to keep a slow and steady flow of blood through 

 the capillaries, that the tissues may be constantly supplied. 



The Veins. The capillaries, after penetrating the tis- 

 sues, unite again to form small veins, which in turn unite 

 to form larger ones, till finally two great veins, the pre- 



Longitudinal Section 



Capillaries, composed of a Single 

 Layer of Cells. 



