CHAPTEE XII 



MECHANICS OF EESPIKATION 



SUMMARY. 1. Historical. 2. Glandular structure of the lungs. 3. Conditions 

 of the lungs and other viscera within the thorax ; passive movements due to 

 variations in the negative thoracic pressure. 4. The thoracic cavity : changes of 

 form and dimensions with inspiratory and expiratory movements. 5. Muscular 

 mechanism of inspiratory and expiratory movements. 6. Normal and forced 

 respiration. 7. Accessory or concomitant respiratory movements. 8. Ventilation 

 or renewal of pulmonary air (spirometry), and respiratory pressure in the air- 

 passages (pneumatometry). 9. Respiratory displacement of the lungs, and 

 acoustic phenomena of percussion and auscultation. 10. Respiratory variations 

 of intra-thoracic and intra-abdominal pressure. 11. Respiratory variations of 

 pressure in the vena cava. 12. Respiratory variations of aortic pressure. 

 13. Effect of respiratory mechanics on the circulation of the blood. 14. Special 

 forms of respiratory movements. Bibliography. 



IN order that the respiratory gas exchanges may be adequate for 

 the needs of ordinary life, it is essential that the air contained in 

 the alveoli of the lungs should be constantly renewed. A slow but 

 continuous replacement of alveolar air occurs by diffusion with 

 the air contained in the respiratory tract, which is, as we have 

 seen, persistently richer in oxygen and poorer in carbonic acid, 

 from the small to the large bronchi, and from these to the trachea. 

 This renewal by diffusion is facilitated by the gentle impacts given 

 to the lungs by the rhythmical movements of the heart (cardio- 

 pneumatic movements, as discussed in Chap. VII. 10). During the 

 physiological lethargy of hibernating animals, and in the profound 

 cataleptic state of apparent death produced under certain morbid 

 conditions, or by hypnotic influences, such as are employed by the 

 Indian fakirs, this occult and silent renewal of the pulmonary air 

 may suffice to maintain life for a long time, since the physiological 

 need of respiration is extraordinarily reduced. But under normal 

 conditions there is a crying want for more energetic replacement 

 of the air by a real pulmonary ventilation, produced by the 

 alternate rhythmical expansion and retraction of the thorax, in 

 which the lungs are hermetically enclosed these organs being 

 eminently elastic, and yet capable of passively following the 

 thorax when it acts as a suction and a pressure pump. 



I. The simplest and most fundamental experiments on the 



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