THE RESPIRATORY SYSTEM 



Respiration is the process whereby the tissues are supplied 

 with oxygen and " relieved of their carbon dioxid. In mam- 

 mals, 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, CO2 (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 di\ddes 

 up into capillaries ramifying over the air sacs (Fig. 95), thus 

 permitting the carbon dioxid to escape into the air sacs. Other 

 waste products resulting from chemical acti\aty 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 mem- 

 brane 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 thyreohyal bone (Fig. 21). Dorsal to the larynx 

 is the esophagus. By removing carefully the muscles and 



191 



