120 PRINCIPLES OP VETERINARY SCIENCE 



of the internal intercostal muscles, which help draw the ribs 

 backward and downward. When breathing is forced or labored, 

 these and the abdominal muscles also come into play to aid in 

 reducing the size of the thorax. 



The air which is exhaled in normal, quiet respiration is known 

 as tidal air; the supplemental air is that which can be voluntarily 

 breathed out after a quiet expiration; the residual air is that which 

 the animal is unable to force out of the lungs. To show that 

 residual air is present in considerable amount, the lungs of a 

 recently killed animal should be placed in water, in which they 

 will be found to float. Fetal lungs sink in water, for the air-cells 

 have never been filled with air. 



Internal respiration, true respiration, or oxidation, consists of 

 the passage of oxygen from the blood to the tissues, and the pas- 

 sage of carbon dioxid and other impurities from the tissues to the 

 blood. The process is one of the fundamental properties of 

 protoplasm and was referred to in Chapter I under Katabolism. 

 The blood is constantly supplying oxygen and removing carbon 

 dioxid. The balance between these gases in the cells is main- 

 tained by the cell protoplasm, which has a marked attraction 

 for oxygen. 



Two theories have been advanced to explain the exchange that 

 takes place between the gases in the alveolar air and the gases 

 in the blood of the pulmonary capillaries, and the corresponding 

 interchange between the blood and the tissue cells. 



One theory is that a simple mechanical diffusion of the gases 

 occurs. It is claimed that diffusion results from the difference 

 in tension, the process being one of passing gases from a place of 

 lower tension to that of a higher. By studying the accompanying 

 tables, which show the average per cent, of the different gases in 

 the inspired and expired air, and in the arterial and venous blood 

 respectively, a conception may be had of the extent and nature 

 of the gaseous exchanges. 



COi 

 O and CHj N 



Inspired air 21.0 0.0 79 



Expired air 16 . 5 4.5 79 



Difference 4.5 4.5 



The above table shows that the respired air loses oxygen and 

 takes up carbon dioxid and methane. Of the latter two gases 



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