188 APPLIED PHYSIOLOGY 



inteiest, for there is no doubt that reflex stimulation 

 of them is the cause of asthma. As has been already 

 mentioned, reflex constriction of the bronchioles can be 

 best obtained by exciting the nasal mucous membrane, 

 but it can also be induced by impulses from other 

 organs e.g., the stomach. Certain drugs have a marked 

 effect on the apparatus. Muscarin, for example, stimu- 

 lates the nerve endings, and may induce an artificial 

 asthma, whilst the inhalation of chloroform or ether, or 

 the injection of atropin, abolishes the effect of the vagus 

 and leads to bronchial relaxation. 



The Mechanics of Respiration. 



The lungs may be compared to two elastic bags 

 partially distended with air and enclosed in a closed box, 

 the thorax. They are only prevented from collapsing 

 altogether by the pressure of the atmosphere (amounting 

 to 15 pounds to the square inch), which can reach their 

 interior through the air passages, but is prevented from 

 pressing on their outer surfaces by the resisting chest 

 wall. When the chest wall is perforated, the lung on 

 that side at once falls in to about one-fifth of its original 

 bulk.* 



This tendency of the lungs to collapse their 'con- 

 tractility, 1 as it may best be called is the result mainly 

 of the large quantity of elastic tissue in the lung 

 substance, but in part also, perhaps, of the tonicity of 

 the muscular fibres in the walls of the bronchi, and has 

 been described by some writers as ' pulmonary tone.'t 



* See Salter, Lecture on Dyspnoea, Lancet, 1865, ii. 111. 

 f Samuel West, Med. Chir. Soc. Trans., 1898, Ixxxi. 273. 



