186 THE MECHANISM OF 



plex space in which a terminal bronchiole ends. In the frog's 

 lung the central space represents the infundibular element of the 

 mammalian lung ; in reptiles it is represented by the thin walled 

 posterior part of the lung ; in birds it is represented by the air 

 sacs ; in the mammalian lung the distensible element is scattered 

 throughout the lung, whereas in all other vertebrate it forms a 

 separate part. If out of a rubber balloon a model of the terminal 

 bronchiole infundibulum and alveoli be made and inflated, it is 

 the central or infundibular space which expands most, the alveoli 

 implanted on its walls being widened but at the same time ren- 

 dered more shallow. The point which one seeks to emphasise 

 is that it is not the alveoli but the infundibula that should be 

 regarded as the essential expansile parts of the lung ; the larger 

 and more plentiful the infundibula in a part of the lung, the more 

 readily will that part respond to any distending force. Oppel 

 gives the diameters of the infundibula in the apical part of the 

 lung as -12 mm. at the third year and -45 mm. at the seventieth ; 

 in the basal part of the lung as -38 mm. at the third year and '85 

 mm. at the seventieth. They are largest in the subpleural zone 

 and smallest in the root rone. One infers, therefore, that, when a 

 breath is taken, the base expands more readily and to a greater 

 extent than the apical part, and the subpleural more than the part 

 at the root. When emphysema occurs it is the infundibula which 

 first become hyperdistended,and the parts most liable to emphysema 

 are those in which the infundibula have normally the greatest size. 



THE BRONCHIAL MUSCULATURE 



The older physiologists regarded the bronchial musculature as 

 expiratory in function ; even now most medical writers ascribe 

 the reflex contraction of the lung (Abram's reflex) which follows 

 any stimulation of the chest wall to the action of the bronchial 

 musculature. It is more probable that the retraction of the lung 

 is due to a reflex contraction of the musculature of the body wall. 

 The action of the musculature of the bronchioles in regulating 

 the tension of the air within the infundibula has not received the 

 attention it deserves. The more the calibre of the bronchiole is 

 diminished during inspiration the greater must be the negative 

 tension within the infundibula, a tension which must act on and 

 distend the capillaries and various blood spaces embodied in the 



