RESPIRATION 181 



and can be seen to consist of two layers one circular, 

 the other longitudinal. The function of the circular 

 layer in controlling the entrance of gases into the air 

 cells, and the inconvenience it may give rise to in the 

 shape of asthma, have already been pointed out. The 

 longitudinal layer probably aids in producing the 

 contractility of the lung (vide p. 188). It is believed, 

 too, by some that the asthmatic attacks which sometimes 

 supervene in the course of chronic bronchitis are due to 

 the inflammatory products which infiltrate the bronchial 

 wall, affecting the delicate longitudinal fibres more than 

 the circular, with the result that the action of the latter 

 is unopposed and spasm supervenes.* 



Each bronchiole terminates in a ' lobule,' which may 

 be regarded as the ultimate pulmonary unit,t or a lung 

 in miniature, shut off by a fibrous covering and endowed 

 with its own vessels, nerves, and lymphatics, and capable 

 of becoming the seat of disease independently of the 

 other lobules by which it is surrounded. In the interior 

 of the lobule the bronchiole subdivides into still smaller 

 divisions (intralobular bronchioles), the terminal branches 

 of which end in the narrow alveolar ducts, which expand 

 in turn into the comparatively wide infundibula lined by 

 their air cells. This sudden widening out of the air 

 passage is believed to aid in the production of the 

 ' inspiratory sound ' heard in ausculting the lungs, eddies 

 being set up in the passage of the air from the narrower 

 to the wider cavity. 



When the lungs expand during inspiration the infun- 



* Aufrecht, Deut. Arch. f. Klin. Med., 1900, Ixvii. 586. 

 f See Ewart, 'The Bronchi,' etc. (Lond.), 1889. 



