GENERAL STRUCTURE OF THE BODY. 27 



nerve becomes tired, degeneration is more rapid. In 

 fact, the degree of exhaustion is determined by several 

 factors, as by relation to the central nervous system, 

 variations in temperature, blood supply, and functional 

 activity, the process being more rapid in warm than in 

 cold blooded animals. 



Cilia. A few motions are accomplished by tissue that 

 is not muscular, as in the case of the cilia attached to the 

 cells of the respiratory tract, which lie flat on the free 

 surface and then lash forward, serving in the air cells to 

 keep the air in motion and in the tubes to send secretions 

 from below upward and outward and to keep out for- 

 eign bodies. Cilia are also found in the female genital 

 tract, where they aid the passage of the ovum from the 

 ovary to the womb. They act together, though appar- 

 ently not governed by the nervous system. As in the 

 white corpuscles of the blood, whose motion also is not 

 muscular, the changes that take place in ciliated epithe- 

 lium are probably about the same as those in muscular 

 tissue, that is, contractile. 



The Blood. To most of the tissues just described 

 nourishment is brought in the blood, which cir- 

 culates through the body in a system of hollow tubes, 

 the arteries and veins, whence it is distributed through 

 the agency of the lymphatic system. There are no blood- 

 vessels, however, in the epidermis, epithelium, nails, 

 hair, teeth, nor in the cornea of the eye. The vessels 

 that carry the blood from the heart are called arteries, 

 those that return it veins. The former begin as large 

 vessels and gradually decrease in size; the latter begin 

 as small vessels and form larger and larger trunks as 

 they approach the heart. 



The arteries have three coats: 1. a thin, serous coat, 

 the internal or intima; 2. a middle or muscular coat, and 

 3. an external coat of connective tissue. The middle 

 coat is the thickest and is the one that prevents the walls 

 from collapsing when cut across. Except in the cra- 

 nium, each artery is enclosed in a sheath with its vein 



