CHAP, xii MECHANICS OF RESPIKATION 403 



mechanics of respiration are those of Galen (A.D. 131-203). He 

 was the first to assert that the lungs passively followed the 

 movements of the thorax, but he assumed that there was a layer 

 of air between the pulmonary walls and the thorax. 



Oribasus (A.D. 360) was the first who noticed the collapse of 

 the lung in double pneuniothorax, and Vesalius (as we have seen, 

 p. 371) the first who employed artificial respiration by a bellows 

 inserted into the opened thorax to maintain life. 



Malpighi( 1061) first described the structure of the lungs. Alfonso 

 Borelli (1679J first formulated a complete theory of the mechanism 

 of pulmonary ventilation. To Haller (1780) belongs, however, the 

 merit of explicitly denying that the pleural cavity contains air a 

 notion which many clung to, producing not a little confusion of ideas. 

 He further asserted the absolute passivity of the movements of 

 the lungs, which some of the earlier physiologists regarded as the 

 primum movens of pulmonary ventilation, while others after him 

 (Rudolph, 1821 ; Laennec, 1819) still believed, on the strength of 

 fallacious or wrongly interpreted observations, that the lungs 

 were capable of active movements independent of the thorax. 



The exact determination of the muscular mechanisms that 

 govern the alternate acts of inspiration and expiration, and the 

 right appreciation of their functional value in normal (eupnoea) 

 and abnormal (dyspnoea) respiratory rhythm, is still the subject of 

 innumerable controversies, as we shall see in the course of the 

 present chapter. 



II. We must leave the full description of the structure of the 

 lungs and air-passages, i.e. the trachea, and the large and small 

 bronchi which lead to the alveoli (where, as we have seen, the gas 

 exchanges between air and blood are carried on), to text-books of 

 anatomy and histology ; it is sufficient here to remark that from 

 the physiological point of view they may be regarded as large 

 branching glands lined with mucosa, in which the ramified 

 bronchial tubes represent the excretory canals, and the infundibuli 

 divided by internal septa into alveoli, or terminal air-cells of 

 somewhat polygonal form, the secreting glands. These air-cells 

 are lined with a single layer of cells, \vhich are characterised by 

 their reduction to thin laminae, some nucleated, others non- 

 nucleated, which accounts for their secretory activity being 

 reduced to a minimum, as suggested tentatively in the previous 

 chapter, or entirely wanting, as held by the great majority of 

 physiologists. In immediate contact with the alveolar epithelium 

 is the network of pulmonary capillaries, with exceedingly fine 

 meshes, in the centre of which lies the denser and nucleated part 

 of the epithelial cells, while the more attenuated marginal cells, 

 which are reduced to a delicate lamella, invest the surface of the 

 capillaries facing the alveoli, so that the capillary network and 

 the epithelium which lines them internally form but a single layer. 



