124 



THE MECHANISM OF THE CIRCULATION. 



withstand the pressure of the atmosphere. If the thoracic cavity he freely 

 opened, it appears to be far larger than its contents, for, at the moment 

 of opening, the lungs collapse. In the foetus, on the other hand, the 

 opened thorax is exactly filled by its contents. When the new-born 

 child takes its first respiration the thoracic walls assume a new position, 

 and the cavity then becomes permanently enlarged. The explanation 

 of this is not quite clear. It has been suggested that the expiratory 

 mechanism is over-stretched by the first inspiration, and is not endowed 

 with sufficient elasticity to pull the chest back into its old position. 1 



In consequence of the enlargement of the thorax, the intrathoracic 

 pressure becomes negative at the first breath of the child, and the lungs 

 are expanded to till the partial vacuum thus formed. The expanding 

 force is the pressure of the atmosphere acting down the trachea. On 

 one side of the pulmonary air-cell is the full pressure of the atmosphere ; 

 on the other side, the sub-atmospheric pressure within the thoracic 



Fig. 78. — Respiratory undulations of aortic and vena cava pressures. 

 Effect of vagotomy and the vertical feet-down position. A-B, 

 vagi divided ; C, feet-down position. The downstroke of the 

 vena cava trace signifies inspiration. — Hill and Barnard. 



cavity. The lungs are elastic bags ; their structure is permeated by 

 elastic tissue ; in virtue of this elasticity, the lungs, when expanded, 

 oppose a force contrary in sign to that of the atmospheric pressure. 

 In consequence of this elasticity, the lungs collapse when the thorax is 

 freely opened. In the natural state the pressure within the thorax is 

 at any moment less than that of the atmosphere by that amount of the 

 atmospheric tension which is required to overcome the elasticity of the 

 lungs and to distend these organs to the size of the thoracic cavity. The 

 intrathoracic pressure is measured by the insertion of a tube through 

 an intercostal space into the cavity of the chest. This tube is connected 

 with a manometer. 



By a series of such determinations Donders 2 arrived at the follow- 

 ing values of the intrathoracic negative pressure or elastic pull of the 

 lungs in man : — 



In the position of ordinary quiet inspiration — 9 mm. Hg. 

 „ „ expiration — 7'5. 



,, deepest inspiration . . — 30 to - 40. 



1 J. Bernstein, Arch. f. d. ges. Physiol., Bonn, 1878, Bd. xvii. S, 617, 

 - Ztsehr.f. rat. Med.', 1853, Bd. iii. S. 287. 



