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A TEXTBOOK OF PHYSIOLOGY 



by the vagus and sympathetic nerves. During hard exercise the 

 capacity of the bronchial tubes is said by some to increase in order 

 to lessen resistance and facilitate the rapid and great ventilation of 

 the lungs. By means of this muscle the airway can certainly be 

 increased or decreased so that air may more or less readily be 

 taken into the lungs (Fig. 146). Drugs also affect this mechanism 

 (Figs. 147, 148). The muscle can control the entry of air into one or 



FIG. 146. DECEREBRATE ANIMAL: LUNG VOLUME AND BLOOD -PRESSURE. TO 

 SHOW EFFECT OF EXCITATION OF PERIPHERAL VAGUS AND OF RIGHT CERVICAL 

 SYMPATHETIC. (Dixon and Ransom.) 



other part of the lung. There is some evidence that all parts of the 

 lung are not in action during quiet breathing. The muscle supports 

 the tubes in expiratory effort. 



The infundibular sacs of the lung are lined with flattened cubical 

 cells, supported by a framework of elastic fibres and richly supplied 

 by bloodvessels. The alveoli are smaller terminal expansions set in 

 the funnel-shaped infundibula. 



Air Volume. In dealing with respiration, certain terms are used 

 which require defining. The volume of air breathed by a person at 

 each quiet respiration is termed the tidal air. This averages, in a 





