118 PRINCIPLES OP VETERINARY SCIENCE 



cock attachment, a negative pressure may be made. If the 

 sheet of rubber at the bottom of the apparatus is pulled down- 

 ward, the size of the cavity will be increased and the pressure 

 still further lowered, causing the air to rush in through the trachea 

 and expand the lungs (L) . When the pull is released, the pleural 

 pressure rises, and the lungs tend to collapse and force the air 

 out. The bell-jar used as a model for our illustration was 

 larger than necessary, and may give a wrong impression, unless 

 it is remembered that in life the lungs and other organs com- 

 pletely fill the thoracic cavity, so that no air space exists between 

 them. 



As the air that is inhaled passes through the nasal cavity, it 

 comes in contact with the warm, moist mucous membrane which 

 lines this passage and is spread out over the extensive surfaces 

 of the turbinated bones. Here it is warmed and moistened so 

 that no irritation occurs when it comes in contact with the sensi- 

 tive lung tissue. 



The ciliated cells in the trachea and bronchi continuously wave 

 their cilia backward and forward to work the mucus, secreted 

 by the large mucous cells previously mentioned, toward the 

 mouth. This prevents the air sacs from clogging with phlegm 

 and dust, as would occur in a little while if the foreign material 

 were not removed. 1 



Respiration is a rhythmic process, that occurs from 8 to 16 

 times per minute in the horse, by which the exchange of gases 

 between the tissues and the surrounding medium is made possible. 

 The process is complex and involves a combination of the proc- 

 esses of absorption, secretion, and excretion. Although we 

 think of the products concerned in respiration as being gases, they 

 really must go into gaseous solution before they are available 

 for use by the body. It will be remembered that food materials 

 are rendered water soluble before they can be absorbed, as are 

 all glandular products that are secreted and excreted, so the 

 process of respiration does not differ materially from those 

 carried on by other systems of organs. Absorption is very 

 rapid from the respiratory mucous membrane for such readily 

 diffusible substances as ether, chloroform, alcohol, and water. 



1 The action of cilia may be demonstrated by the "cork experiment" 

 on the frog. 



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