660 PHYSIOLOGY OF RESPIRATION. 



passages are open, the intrathoracic pressure is still negative by a 

 small amount, since the lungs are still expanded beyond what 

 might be called their normal size, that is, their size when the pres- 

 sure inside and outside is the same. If, however, a forced expira- 

 tion is made with the glottis closed, as in the straining movements 

 of defecation, parturition, etc., then naturally the intrathoracic 

 pressure rises with the intrapulmonary pressure. The increased 

 pressure from the compressed air in the lungs is felt upon the organs 

 in the mediastinal spaces. The large veins especially are affected, 

 and the flow in them is partially blocked, as is shown by the swelling 

 of the veins in the neck outside the thorax. The maintenance of 

 such conditions for a considerable period may seriously affect the 

 circulation. The same general effect is obtained also in attacks of 

 coughing, the violent spasmodic expirations with closed glottis 

 causing a visible venous congestion in the head from the obstruction 

 to the venous flow into the heart. Forcible inspirations, on the 

 other hand, lower the intrathoracic pressure that is, increase the 

 negativity whether the glottis is open or closed. When the glottis 

 is freely open and a ' deep inspiration is made the intrathoracic 

 pressure may fall as much as 30 mms. Hg, that is, become equal 

 to 730 mms. The lungs being much more expanded exert a corre- 

 spondingly greater elastic force. If the glottis is closed during a 

 deep inspiration then there is little actual expansion of the lungs, 

 but the intrapulmonary pressure falls from the rarefaction of the air 

 in the lungs, and the intrathoracic pressure, of course, falls with it. 

 The Origin of the Negative Pressure in the Thorax. As is evi- 

 dent from the above explanation, the fact that the pressure in the 

 thorax is less than an atmosphere is due in the long run to the 

 circumstance that the lungs are smaller than the thoracic cavity 

 which they occupy. In the fetus the lungs are solid, and completely 

 fill the thoracic cavity, except for the part occupied by the other 

 organs. It has been a question whether after birth the size of the 

 thoracic cavity is suddenly and permanently increased by the first 

 inspiratory movements, and a negative intrathoracic pressure thus 

 produced at once. The careful experiments of Hermann* seem to 

 have settled this point. He proved that newly-born children 

 between the first and the fourth day, show no measurable negative 

 pressure in the thorax, and at the eighth day the pressure in the 

 thoracic cavity is less than atmospheric by an amount equal to only 

 0.4 mm. Hg. The negative pressure as we find it in the adult is 

 evidently developed gradually, and is due to the fact that the 

 thorax increases in size more rapidly and to a greater extent than 

 the lungs, so that to fill the cavity the lungs become more and more 

 expanded. It follows, also, from these facts, that the new-born 

 * Hermann, " Archiv f. d. gesammte Physiologie, " 30, 276, 1883. 



