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 trans- 

 ferred through the veins to the heart and thence to the 

 lungs. Here the pulmonary artery divides up into capil- 

 laries ramifying over the air sacs (Fig. 84), thus per- 

 mitting 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 cranial 

 end of the trachea, at the base of the tongue. The basihyal 

 bone is attached to the cranial ventral margin of the larynx 

 and on each side is a thyrohyal bone (Fig. 20). Dorsal 



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