64 



RESPIRATORY MECHANISMS 



to the pleura. The elastic force resides mainly in a network 

 of fibres, but the alveolar walls themselves and the capillary 

 vessels, made up of epithelial cells only, are also highly dis- 

 tensible and elastic. By inflation of the lungs the surface area 

 therefore becomes increased and the thickness of the mem- 

 branes separating the blood 

 from the air correspondingly 

 diminished. 



Experiments on the rate 

 of diffusion of carbon mon- 

 oxide into the lungs of man 

 (Marie Krogh, 1912) make 

 it probable that below a cer- 

 tain volume, corresponding 

 to the normal expiratory 

 position, the alveolar surface 

 and epithelial thickness be- 

 come constant by folding of 

 the wall. It is the rule 

 that the finer bronchi have 

 smooth muscle fibres in the 

 wall, regulating the distribu- 

 tion of the air, and in many 

 reptiles muscle fibres are present in the pulmonary tissue 

 proper, and rhythmic contractions can take place the function 

 of which is unknown. 



The ventilation mechanism shows a characteristic evolution. 

 In the air-breathing Amphibia the lungs are filled under pres- 

 sure from the bucco-pharyngeal cavity by acts of swallowing 

 and, when the glottis is kept open without such swallowing, the 

 air flows out until the lungs are collapsed. In some reptiles 

 the swallowing mechanism persists and is sometimes used in 

 emergency or (e.g., in Chamceleo) to inflate the lungs and air- 

 sacs to a very large volume, but the normal mechanism in this 

 group and in all the warm-blooded animals is inspiration by 

 active suction. This means that the lungs are placed in a 

 closed pleural space which can be expanded by muscular 



Fig. 38. Diagram of alveolar unit 

 in man opening from final bronchus. 

 L.a. branch of pulmonary artery. B.a. 

 bronchial artery. L.v. pulmonary vein. 

 Ca. 25/1. (Stohr.) 



