1 66 MECHANISM OF RESPIRATION. 



the parietal pleura, it is clear that the lungs must be distended with every dilatation 

 of the chest, and diminished by every contraction thereof. The movements of the 

 lungs, therefore, are entirely passive, and are dependent on the thoracic movements. 



On account of their complete elasticity and their great extensibility, the lungs 

 are able to accommodate themselves to any variation in the size of the thoracic 

 cavity, without the two layers of the pleura becoming separated from each other. 

 As the capacity of the non-distended chest is greater than the volume of the 

 collapsed lungs after their removal from the body, it is clear that the lungs, even 

 in their natural position within the chest, are distended, i.e., they are in a certain 

 state of elastic tension ( 60). The tension is greater the more distended the 

 thoracic cavity, and vice versd. As soon as the pleural cavity is opened by perfora- 

 tion from without, the lungs, in virtue of their elasticity, collapse, and a space filled 

 with air is formed between the surface of the lungs and the inner surface of the 

 thoracic wall (pneumo-thorax). The lungs so affected are rendered useless for 

 respiration ; hence a double pneumo-thorax causes death. 



Pneumo-thorax. It is also clear that, if the pulmonary pleura be perforated from within the 

 lung, air will pass from the respiratory passages into the pleural sac, and also give rise to 

 pneumo-thorax. [Not un frequently the surgeon is called on to open the chest, say by removing 

 a portion of a rib to allow of the free exit of pus from the pleural cavity. If this be done with 

 proper precautions, and if the external wound be allowed to heal, after a time the air in the 

 pleural cavity becomes absorbed, the collapsed lung tends to regain its original form, and again 

 becomes functionally active. ] 



Estimation of Elastic Tension. If a manometer be introduced through an intercostal space 

 into the pleural cavity, in a dead subject, we can measure, by means of a column of mercury, 

 the amount of the elastic tension required to keep the lung in its position. This is equal to 

 6 mm. in the dead subject, as well as in the condition of expiration. If, however, the thorax 

 be brought into the position of inspiration by the application of traction from without, the 

 elastic tension may be increased to 30 mm. Hg. (Donders). 



If the glottis be closed and a deep inspiration taken, the air within the lungs must 

 become rarefied, because it has to fill a greater space. If the glottis be suddenly 

 opened, the atmospheric air passes into the lungs until the air within the lungs has 

 the same density as the atmosphere. Conversely, if the glottis be closed, and if an 

 expiratory effort be made, the air within the chest must be compressed. If the 

 glottis be suddenly opened, air passes out of the lungs until the pressure outside and 

 inside the lung is equal. As the glottis remains open during ordinary respiration, 

 the equilibration of the pressure within and without the lungs will take place gradu- 

 ally. During tranquil inspiration there is a slight negative pressure ; during ex- 

 piration a slight positive pressure, in the lungs ; the former = 1 mm., the latter 2-3 

 mm. Hg. in the human trachea (measured in cases of wounds of the trachea). 



108. aUANTITY OF GASES RESPIRED. As the lungs within the chest 

 never give out all the air they contain, it follows that only a part of the air of the 

 lungs is changed during inspiration and expiration. The volume of this air will 

 depend upon the depth of the respirations. 



COMPLEMENTAL 

 AIR 

 110 



TIDAL AIR 

 20 



<5 >' 



CO 



g Hutchinson denned the following : 



gs % (1) Residual air is the volume of air which remains in the 



op chest after the most complete expiration. It is =1230-1640 c.c. 

 [100-130 cubic inches]. 



(2) Reserve or supplemental air is the volume of air which 



< can be expelled from the chest after a normal quiet expiratioD. 



g It is =1240-1800 c.c. [100 cubic inches]. 



Jf jja (3) Tidal air is the volume of air which is taken in and given 



" o out at each respiration. It is =500 cubic centimetres [20 cubic 



h inches]. 



residual air (4) Complemental air is the volume of air that can be forcibly 

 100 inspired over and above what is taken in at a normal respiration. 

 1 It amounts to about 1500 c.c. [100-130 cubic inches]. 



RESERVE AIR 



100 



(5) Vital Capacity is the term applied to the volume of air which can be 



