402 RESPIRATION. [CHAP. XXIX. 



to modify the relation which the inspiratory and expiratory powers 

 bear to each other. Thus, wrestlers, boxers, and others, accus- 

 tomed to employ the extraordinary muscles of respiration, in ren- 

 dering the thorax a fixed point from which other muscles might 

 act, were found to have an unusual power of expiration. In one 

 wrestler he found the expiratory power exceed nearly four times 

 that of the natural inspiratory power. Valentin in six experiments 

 on young adult males found the force exerted in ordinary) tranquil 

 inspiration and expiration to be from about one-seventh to three- 

 sevenths of an inch of mercury. Dr. Hutchinson calculates that a 

 man who raises three inches of mercury by an effort of inspiration 

 exerts a force equal to 1000 Ibs. In one man he found the mercury 

 raised to such a height (seven inches) as to indicate a force of 

 2200 Ibs., or nearly a ton. 



When the capacity of the thorax is augmented, the air in the 

 lungs expands so as to keep them in contact with the walls, and the 

 external air enters by the trachea to restore the equilibrium. When 

 the dilating force ceases, the walls of the chest contract upon the 

 lungs, and expel a portion of the air, the lungs themselves con- 

 spiring. 



Use of the Tracliealis Muscle. That the tracheal is in contracting 

 must diminish the area of the trachea and bronchia is obvious from 

 its arrangement no less than from experiment. Dr. Williams found 

 that the lungs taken from an animal just killed manifestly con- 

 tracted under the stimulus of galvanism. The fibres are allied to 

 those of the ducts of glands, and of the blood-vessels, and are 

 consequently probably incapable of coinciding in action with the 

 muscles of respiration. It further seems impossible to assign an 

 object to any peristaltic action which might be attributed to them. 

 We are disposed to consider that the trachealis muscle contracts 

 and dilates the air-tubes slowly, in relation to the activity of the 

 respiratory function at different times of the day, or under other 

 modifying circumstances. Its contraction would tend to limit the 

 quantity of air transmitted in a given time, or would quicken its 

 passage. It is in all probability contracted spasmodically in spas- 

 modic asthma, and in the early stages of bronchitis. If it contracts 

 during the sensation that precedes and accompanies coughing, this 

 would facilitate the expulsion of mucus by quickening the expelled 

 current of air. 



Excitation of the Respiratory Movements. We have already re- 

 ferred to the share taken by the nerves in respiration. The respi- 

 ratory acts are essentially involuntary, and unconsciously performed, 



