CHAPTER XIII. 

 RESPIRATION. 



Respiration is a process by which oxygen is introduced into, and 

 carbon dioxid removed from, the body. The assimilation of the 

 former and the evolution of the latter take place in the tissues as a 

 part of the general process of nutrition. Without a constant supply 

 of oxygen and an equally constant removal of the carbon dioxid, 

 those chemic changes which underlie and condition all life phenom- 

 ena could not be maintained. 



The general process of respiration may be considered under the 

 following headings, viz. : 



1. The structure and general arrangement of the respiratory ap- 



paratus. 



2. The mechanic movements of the thorax by which an interchange 



of atmospheric and intra-pulmonary air is accomplished. 



3. The chemistry of respiration, the changes in composition ex- 



perienced by the air, blood, and tissues. - 



4. The nerve mechanism by which the respiratory movements are 



maintained. 



THE RESPIRATORY APPARATUS. 



The respiratory apparatus consists essentially of: 



1. The lungs and the air-passages leading into them: viz., the nasal 



chambers, mouth, pharynx, larynx, and trachea. 



2. The thorax and its associated structures. 



The nasal chambers are the natural entrances for the inspired 

 air. Their complicated structure slightly retards the movement of 

 the air, in consequence of which its temperature and moisture are 

 adjusted to the physiologic conditions of the lower respiratory pas- 

 sages. The mouth, though frequently serving as an entrance for air, 

 is not primarily a respiratory passage. Both the nasal chambers 

 and the mouth communicate posteriorly with the pharynx, in which 

 the respiratory and the deglutitory passages cross each other, the 

 former leading directly into the larynx. 



The larynx is a complicated mechanism serving the widely 

 different though related functions of respiration and phonation. It 

 consists of a framework of cartilages, articulating one with another, 

 united by ligaments and moved by muscles; it is covered externally 

 with fibrous tissue and lined with mucous membrane. The superior 



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