3S 6 RESPIRATION. [CH. xxiv. 



affections, much more commonly, that the number of the pulse- 

 beats is greater in proportion than that of the respirations. 



The Force of Inspiratory and. Expiratory Muscles. The force 

 with which the inspiratory muscles are capable of acting is 

 greatest in individuals of the height of from five feet seven 

 inches to five feet eight inches, and will elevate a column of 

 nearly three inches (about 60 nun.) of mercury. Above this 

 height the force decreases as the stature increases ; so that the 

 average of men of six feet can elevate only about two and a half 

 inches of mercury. The force manifested in the strongest expira- 

 tory acts is, on the average, one-third greater than that exercised 

 in inspiration. But this difference is in great measure due to the 

 power exerted by the elastic reaction of the walls of the chest; 

 and it is also much influenced by the disproportionate strength 

 which the expiratory muscles attain, from their being called into 

 use for other purposes than that of simple expiration. The force 

 of the inspiratory act is, therefore, better adapted than that of 

 the expiratory for testing the muscular strength of the body. 

 (John Hutchinson.) 



In ordinary quiet breathing, there is a negative pressure of 

 only i mm. during inspiration, and a positive pressure of from 

 2 to 3 mm. mercury during expiration. 



The instrument used by Hutcliinson to gauge the inspiratory and expira- 

 tory power was a mercurial manometer, to which was attached a tube 

 fitting the nostrils, and through which the inspiratory or expiratory effort 

 was made. 



The greater part of the force exerted in deep inspiration is 

 employed in overcoming the resistance offered by the elasticity of 

 the lungs. 



In man the pressure exerted by the elasticity of the lungs alone 

 is about 6 mm. of mercury. This is estimated by tying a mano- 

 meter into the trachea of a dead subject, and observing the rise of 

 mercury that occurs on puncture of the chest-walls. If the chest 

 is distended beforehand so as to imitate a forcible inspiration, a 

 much larger rise (30 mm.) of the mercury is obtained. In the 

 body this elastic force is assisted by the contraction of the plain 

 muscular fibres of the alveoli and bronchial tubes, the pressure of 

 which probably does not exceed i or 2 mm. Hutchinson calculated 

 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. 



It is possible that the contractile power which the bronchial 

 tubes and air-vesicles possess, by means of their muscular fibres, 

 may assist in expiration ; but it is more likely that its chief 

 purpose is to regulate and adapt, in some measure, the quantity 



