CONTRACTION OF BRONCHI. 217 



Much of the force exerted in inspiration is employed in 

 overcoming the resistance offered by the elasticity of the 

 walls of the chest and of the lungs. Mr. Hutchinson esti- 

 mated the amount of this elastic resistance, by observing 

 the elevation of a column of mercury raised by the return 

 of air forced, after death, into the lungs, in quantity equal 

 to the known capacity of respiration during life ; and he 

 calculated that, in a man capable of breathing 200 cubic 

 inches of air, the muscular power expended upon the elas- 

 ticity of the walls of the chest, in making the deepest 

 inspiration, would be equal to the raising of at least 

 301 pounds avoirdupois. To this must be added about 

 150 Ibs. for the elastic resistance of the lungs themselves, 

 so that the total force to be overcome by the muscles in 

 the act of inspiring 200 cubic inches of air is more than 

 450 Ibs. 



In tranquil respiration, supposing the amount of 

 breathing air to be twenty cubic inches, the resistance of 

 the walls of the chest would be equal to lifting more than 

 100 pounds; and to this must be added about 70 pounds 

 for the elasticity of the lungs. The elastic force overcome 

 in ordinary inspiration must, therefore, be equal to about 

 170 pounds. 



It is probable, that in the ordinary quiet respiration, 

 which is performed without conciousness or effort of the 

 will, the only forces engaged are those of the inspiratory 

 muscles, and the elasticity of the walls of the chest and 

 the lungs. It is not known under what circumstances the 

 contractile power which the bronchial tubes possess, by 

 means of their organic muscular fibres, is brought into 

 action. It is possible, as Dr. E. Hall maintained, that 

 it may exist in expiration; but it is more likely that 

 its chief purpose is to regulate and adapt, in some measure, 

 the quantity of air admitted to the lungs, and to each part 

 of them, according to the supply of blood. Another pur- 

 pose probably served by the muscular fibres of the bron- 



