THE RESPIRATORY SYSTEM. 



Respiration is the process whereby the tissues are sup- 

 plied with oxygen and relieved of their carbon dioxid. In 

 mammals, the special organs of respiration are the lungs, 

 wherein the carbon dioxid is received from the blood, 

 while at the same time the oxygen of the air passes 

 through the thin-walled capillaries to the red blood- 

 corpuscles capable of conveying it to the cells throughout 

 the body. Each cell is composed largely of carbon, 

 hydrogen, oxygen, and nitrogen, and any activity on the 

 part of the cell is the result of the chemical union of 

 some of its elements, whereby several waste products are 

 formed, one of which is the gas, CO 2 (carbon dioxid). 

 This gas is a poison and therefore must be eliminated. 

 It passes through the thin walls of the capillaries adjacent 

 to every cell, and is transferred through the veins to the 

 heart and thence to the lungs. Here the pulmonary 

 artery divides up into capillaries ramifying over the air 

 sacs (Fig. 82), thus permitting the carbon dioxid to 

 escape into the air sacs. Other waste products resulting 

 from chemical activity within the cells are carried away 

 by the kidneys and sweat glands. 



The respiratory system consists of the nasal passages, 

 pharynx, larynx, trachea, and lungs. The air taken in at 

 the anterior nares is warmed in passing over the mucous 

 membrane of the turbinated bones, after which it goes 

 on through the posterior nares (Fig. 18) to the pharynx, 

 and thence into the larynx. 



The larynx is the cartilaginous expansion of the cephalic 



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