238 ANATOMY AND PHYSIOLOGY 



We have seen how the blood returns from the digestive organs 

 laden with food which is to be distributed throughout the body, 

 where the products of digestion are made over in the tissues by a 

 series of changes in which oxygen plays an essential part. The 

 source of the oxygen for this purpose is the air we breathe. It 

 passes through the air vesicles and the capillary walls into the 

 blood, thence into the lymph spaces and tissue cells. 



The gas called carbon dioxide (resulting from tissue action) 

 is brought by the blood to the lungs; passing through the capillary 

 walls and the air vesicles, it is exhaled in the breath and thus re- 

 moved from the body. Consequently, respiration is a process not 

 only of nutrition but of elimination as well. 



This interchange is accomplished in part by the physical process of diffu- 

 sion of gases. (The epithelium of the air vesicles is thought to have a special 

 function o this end, and is called respiratory epithelium.} 



By inspiration we take air, with its oxygen, into the lungs; by 

 expiration we expel it with carbon dioxide, small quantities of 

 ammonia and organic waste matter, and moisture. 



This important process is made possible by the movements of 

 the thorax as follows: 



In inspiration. The external intercostal muscles elevate the 

 ribs and spread them apart, increasing the width of the chest; 1 

 the diaphragm contracts, pulling down its central tendon and thus 

 increasing the depth of the chest; the lungs expand and receive the 

 in-drawn air. This is the active phase of a respiration. 



In expiration. The ribs fall easily back into place, assisted 

 by internal intercostals and abdominal muscles; the diaphragm 

 relaxes, returning to its dome shape, and the air is pressed out. 

 This is the passive phase of a respiration. 



These acts are performed, in health, with regularity, that is, 

 rhythmically. The number of respirations in a moment varies 

 from about 40 in the newly born to 18 in the adult. Normal 

 respiration is slowest when one is lying down or when sleeping. 

 The rate is increased during physical exercise or by emotion, and 

 in visceral inflammations, as pneumonia, pleurisy, peritonitis, etc., 

 also in fevers generally. 



1 The Pectoralis Major and some others assist in deep breathing or forced 

 inspiration. 



