242 THE HUMAN BODY. 



breathe-out, the thorax is diminished and air driven out 

 of the lungs. 



Inspiration and Expiration. The process of taking air 

 into the lungs is known as inspiration, that of expelling it 

 as expiration. On the average, fifteen to eighteen inspira- 

 tions and expirations occur in each minute. We therefore 

 breathe in and out about once for every four beats of the 

 heart. 



The Structure of the Thorax. The thorax is a conical 

 cavity with a rigid supporting skeleton (Fig. 68) formed by 

 the dorsal vertebrae behind, the breast bone in front, and 

 the ribs and rib cartilages on the sides. Between and over 

 these lie muscles, and the whole is covered air-tight by the 

 skin outside and the pleura (p. 238) inside. Above, it is 

 closed by the muscles and other organs of the neck; and 

 below by a movable bottom, the diaphragm. The air-tight 

 chamber thus bounded can be enlarged in all three diame- 

 ters, but especially in the vertical, and in that running 

 from the spinal column to the breastbone (dorso-ventral 

 diameter). 



The Vertical Enlargement of the Thorax. This is 

 brought about by the contraction of the diaphragm, which 

 (as may be seen in Fig. 69), is a thin sheet of muscle, with 

 a fibrous membrane in its centre serving as a tendon. In rest 

 the diaphragm is dome-shaped, its concavity being turned 

 towards the abdomen. From the tendon on the crown of 



What when we breathe-out? 



What is inspiration? Expiration? How often does each occur in 

 a minute? Compare the rate of respiration and of the heart-beat? 



What is the form of the thorax? What forms its skeleton? What 

 lie between and over its bones and cartilage? How is it closed air- 

 tight? 



How is the chest closed above? How below? In what diameters 

 can the chest cavity be enlarged? 



Describe the diaphragm. 



