406 PHYSIOLOGY CHAP. 



and cannot act upon the surface of the lungs (these being her- 

 metically enclosed in the thoracic cavity, formed mainly of rigid 

 walls), that determines the passive distension as well as the, 

 elastic tension of the same, and the negative pressure within the 

 thoracic cavity in the expiratory position. 



Normally the thoracic cavity is completely filled by the lungs, 

 which adapt themselves perfectly to its conformation. The two 

 layers, visceral and parietal, of the pleura, are in immediate 

 contact, separated only by the thinnest stratum of lymph, which 

 facilitates the gliding of one over the other. The pleural cavities 

 are potential only : under morbid conditions they are formed by 

 the liquid or 'solid exudation that is poured out between the two 

 layers ; or by a unilateral or bilateral, external or internal, aperture 

 in the same. The rapid death by asphyxia in double pneumo- 

 thorax shows the passivity of the lung movements, owing to which 

 ventilation or renewal of the air essential to the life of the animal 

 ceases. When, however, from the partial or complete occlusion 

 of the opening, the air cannot freely enter or leave the pleura! 

 cavities, the lungs are still able to distend ; this explains why in 

 many cases of perforation of the thorax the respiratory gas 

 exchanges are not profoundly modified. 



Besides the lungs, the thoracic cavity also contains the heart 

 with the large venous and arterial trunks, which, as hollow organs, 

 indirectly feel the effects of the atmospheric pressure acting 

 directly upon the extrathoracic vessels that communicate with the 

 heart. Intrathoracic negative pressure accordingly determines 

 not merely the distension of the lungs, but also that of the heart 

 and intrathoracic vessels, in proportion with their capacity for 

 dilatation. The thick-walled ventricles of the heart, and the 

 arteries, which are always under strong internal pressure, feel 

 little or no effect from the negative intrathoracic pressure ; the 

 auricles and large trunks, on the contrary, which have thin walls, 

 and are not distended by positive internal pressure, suffer a certain 

 degree of expansion, by which the lumen is widened, and the 

 course of the blood from the extrathoracic to the intrathoracic 

 veins facilitated. 



The oesophagus, as a hollow intrathoracic organ communicating 

 with the exterior, should dilate to a certain extent in consequence 

 of the negative pressure that obtains within the thorax ; its cavity, 

 however, is potential, and is only formed when the canal is 

 traversed by foreign bodies, such as food. Under ordinary con- 

 ditions the walls of the oesophagus are in contact, so that the 

 lumen is obliterated, and the negative intrathoracic pressure, far 

 from aspirating air into the canal, only makes the walls adhere 

 more closely, in consequence of the atmospheric pressure exerted 

 externally upon its intrathoracic portion. But if a hollow 

 sound is passed through the oesophagus, it is converted into 



