170 



ANATOMY FOR NURSES 



[Chap. X 



and narrowing of the arteries not only affects the local circulation in 

 different parts of the body, but the amount of resistance they op- 

 pose to the arterial impulse also influences in some degree the 

 character of the heart-beat. The terra " tone of the arteries " is 

 used to express the normal degree of contracture of the arterial 

 walls. 



Sheaths of the arteries. — The greater number of the arteries 

 are accompanied by a nerve and one or two veins and surrounded 

 by a sheath of connective tissue, which helps to support and hold 

 these structures in position. 



Size of the arteries. — The largest arteries in the body, the 

 aorta and pulmonary artery, measure about one inch (28 mm.) in 

 diameter, at their connection with the heart. These arteries 

 give oft" branches, which divide and subdivide into smaller branches. 

 A branch of an artery is always less than the trunk from which it 

 springs, hence the arteries grow smaller as they subdivide, and 

 gradually lose their characteristic structure. The smallest arte- 

 ries are called arterioles, and at their distal ends, where only the 

 internal coat remains, the capillaries begin. 



CAPILLARIES 



The capillaries are exceedingly minute vessels which average 

 about 2~oVo of an inch (0.0125 mm.) in diameter. They connect 



the arterioles with the venules (smallest 

 veins), thus receiving the blood from 

 the arterioles and carrying it to the 

 venules. 



Structure. — The walls of the capil- 

 laries are formed entirely of 07ie layer 

 of simple endothelium composed of flat- 

 tened cells joined edge to edge by 

 cement substance, and continuous with 

 the laver which lines the arteries and 



veins. 



Distribution. — The capillaries com- 

 municate freely with one another and 

 form interlacing networks of variable 

 form and size in the different tissues. All the tissues, with the 

 exception of the cartilages, hair, nails, cuticle, and cornea of the 



Fig. 106. — Fine Capil- 

 laries FROM THE Mesentery 

 (Collins.) 



