RESPIRATORY CHAMBERS 137 



reached, then you must picture a miniature oak in full 

 summer leaf, with trunk and branches hollowed out so as 

 to form tubes right to the finest terminals which support 

 the leaves. The trunk stands for the bronchus, the great 

 branches for its main divisions, the smaller branches for 

 the secondary divisions, and the terminal twigs which 

 support the leaves for those fine air-pipes known as 

 bronchioles. They lead to the respiratory chambers — 

 represented by the leaves — where the real work of the 

 lungs is carried on. All of these passages — except the 

 terminal bronchial pipes — have hoops or rings of cartilage 

 set in their walls to keep them open — much as soft 

 rubber pipes may be kept open by having a spiral wire 

 wound within them. All the air-pipes can be widened 

 or narrowed, for on their walls are set muscle fibres of 

 the involuntary kind. They relax or contract automat- 

 ically according to the needs of the animal machine, and 

 thus the pipes are widened or narrowed. It is, however, 

 in the terminal bronchioles, leading into the respiratory 

 chambers, that the muscular fibres become really im- 

 portant. If one of these terminal pipelets were to gape 

 while all its. neighbours remained small, then the respi- 

 ratory chamber connected with the wide mouth would 

 receive an unfair share of the incoming tide of air. 

 Hence we find that the terminal twigs of the respiratory 

 tree have not only flaccid walls, unstrengthened by 

 cartilage, but are surrounded by cuffs of muscular fibres, 

 just as we saw was the case with terminal arteries before 

 they open into the capillary fields. The bronchioles of 

 the lung are provided with a most delicate stopcock 

 mechanism to secure an even distribution of the respi- 

 ratory tide. That mechanism is regulated from nerve 

 centres placed in the medulla. There is no need for 

 securing an even distribution of the respiratory tide which 

 enters the cylinder of the engine of a motor cycle. That 

 is because there is only one chamber or cylinder ; if there 

 were millions of chambers, as in the human lung, then 

 a mechanism for just distribution of the air would be 

 necessary. 



