300 PULMONARY MUCOUS TISSUE. 



tracts, the former contracts likewise. The pleura! folds 

 inside these two organs act by means of adhesion by the 

 vacuum, in short, producing a sort of suction, resembling that 

 of cupping-glasses. It therefore appears surprising that what 

 takes place in a cupping-glass, and what physical laws would 

 seem to render necessary, does not take place here, that is, 

 an extravasation of blood or of serum, or permanent effusion. 

 In studying the general physiology of the epitheliums (p. 193), 

 however, we stated that this globular lining had the power 

 to prevent such exudations ; and we find, in this case, an 

 epithelium to which this function is assigned. Pathological 

 observations confirm this view of the matter: it has been 

 observed that nearly all diseases of the pleura, in which effu- 

 sion appears, are caused by more or less entire destruction of 

 the epithelium, or by a state of degeneration which interferes 

 with the exercise of its natural function. 



C. Function of the air-passages in respiration. 



The air, being drawn, by the respiratory movements, into 

 the lung, and then driven out of it, passes through the narrow 

 portion of our pulmonary cone ; that is to say, the nostrils, 

 the nasal chambers, the pharynx, and the trachea with the 

 larynx. All these tubes exhibit mechanical phenomena, 

 accessory to those which we have just been studying in the 

 lung. 



The nostrils dilate actively, but only in deep inspirations, 

 and when there is any sensation of dyspnoea; the nasal 

 chambers exhibit no special mechanical phenomena ; but we 

 know that they perform an important office, as being the 

 place where the inhaled air is prepared, by being charged 

 with heat and steam. 



On a level with the pharynx the air-tube crosses the ali- 

 mentary canal; we saw, in studying the latter, how the 

 upper and lower orifices are obliterated as the food passes 

 (p. 225). 



In some animals the communication between the air-tube 

 and the alimentary canal are permanently closed: in the 

 cetaceans the trachea communicates directly with the nasal 

 chambers, through which alone the animal can breathe. In 

 the case of the pachydermata, the velum of the palate forms 

 at the larynx a half-ring; and the respiration is, consequently, 

 exclusively nasal. The horse, too, breathes only through the 

 nose, on account of the disposition of the velum of the palate 

 and of the epiglotis, the latter reaching to the posterior 



