THE MECHANICS OF THE RESPIRATORY MOVEMENTS 455 



and oral cavities by a relatively narrow tube, known as the 

 trachea. This almost air-tight compartment of the thorax possesses 

 a conical outline, its tip being situated at the root of the neck and its 

 base at the diaphragm. Its ventral wall is formed by the sternum and 

 adjoining costal cartilages, its sides by the ribs, and its dorsal wall by 

 the vertebral column. 



The two lungs occupy almost the entire thoracic cavity, only its 

 central extent being allotted to the heart and large blood-vessels 

 with their pericardial investment. Each organ is closely enveloped by 

 a delicate membrane which is reflected from the bronchi and lines 

 the entire internal surface of the chest wall. Consequently, this 

 membrane which is known as the pleura, consists of two layers, an 

 outer or parietal and an inner or visceral. The opposing surfaces of 

 these layers are lined with flattened endothelial cells and are moistened 

 with a lymphatic secretion to prevent frictioning. It must be em- 

 phasized, however, that the layers of the pleura always remain in close 

 contact with one another and that an actual pleural cavity cannot be 

 present so long as the walls of the chest remain intact. Under normal 

 conditions, therefore, the visceral and parietal layers of the pleura 

 act as one membrane which is interposed between the chest wall and 

 the substance of the lung to facilitate the movement coincident with the 

 expansion of this organ. Between the left and right pleural sacs is a 

 space, known as the mediastinum, which is divided into an anterior 

 and a posterior compartment by the heart with its pericardial in- 

 vestment. In some animals, such as the rabbit, this interpleural 

 space is broad, so that it is possible to expose the heart through the 

 median line of the sternum without rupturing the pleural sacs. But 

 a procedure of this kind is not feasible in most mammals, because the 

 anterior borders of the lungs extend almost to the median line of the 

 thorax. 



THE RESPIRATORY CYCLE 



The muscular act by means of which the air in the pulmonary 

 passages is constantly kept in a fresh state, consists in an alternate 

 increase and decrease in the size and capacity of the thorax which in 

 turn results in a corresponding alteration in the size of the lungs. 

 In spite of their relative solidity, the walls of the chest are flexible so 

 that they may be moved either away from or toward a common center. 

 The former movement takes place during inspiration and the latter 

 during expiration, and naturally, as the intrapulmonary passage 

 stands in communication with the outside through the trachea, an 

 inflow of atmospheric air must result during the expansion of the lung 

 and an outflow during its subsequent period of recoil. Concurrently, 

 it may rightly be inferred that the outward movement of the chest 

 wall may be greatly restricted by obstructing the trachea, because this 

 would prevent the inflow of air and hence, nullify the volumetric in- 

 crease in the capacity of the chest which in turn gives rise to the expan- 

 sion of the lungs. 



