56 



HUMAN BIOLOGY 



Fig. 57. — Capillaries Magni- 

 fied, showing Cells forming 

 their walls. Notice that each cell 

 has a nucleus and three branches. 



leg muscles get more ; after a hearty dinner, the stomach and intestines 

 get more than any other part of the body. Why is it difficult to do the 



best studying and digest a meal at the 

 same time? We see that the muscu- 

 lar coat of the arteries is a very useful 

 coat, for it enables the supply of blood 

 to be increased in any organ which is 

 in temporary need of it. 



Why the Blood Vessels must be 

 Smooth. — The inner coat of the heart 

 and other blood vessels is made of 

 tissue like the epithelial tissue which 

 forms the epidermis and the smooth 

 lining of the mouth and other organs. 

 The purpose of this lining is to lessen 

 friction, and thus save the work of 

 the heart. The friction is greatest in 

 the capillaries because of their small 

 size. The inner coat of smooth cells 

 is the only coat that is prolonged to 

 form the capillaries (see Fig. 57). 



The capillaries are small, thin, short, and very numerous. 

 They arc very small so that they may go in between the 

 cells of the tissues. The capillaries are very thin so that 

 the nourishment from the blood may pass readily into the 

 tissues, and the waste material pass readily into the blood. 

 They are very short so that the friction may be less ; and 

 they are very numerous so that all parts of the tissues may 

 be supplied with blood, and that the blood may flow very 

 slowly through them. Because of the number of the cap- 

 illaries, their total volume is several hundred times larger 

 than the volume of the arteries that empty into them, or 

 of the veins that flow from them. Hence the blood 

 flows slowly through the capillaries, as water flows slowly 

 through a lake along the course of a river. All the 

 changes between the blood and the lungs, and between 

 the blood and the tissues, take place in the capillaries, and 



