360 RESPIRATION [CH. XXTI. 



It varies also much according to different circumstances, such as 

 exercise or rest, health or disease, etc. Variations in the number of 

 respirations correspond ordinarily with similar variations in the 

 pulsations of the heart. In health the proportion is about 1 to 4, 

 or 1 to 5, and when the rapidity of the heart's action is increased, 

 that of the chest movement is commonly increased also ; but not in 

 every case in equal proportion. It happens occasionally in disease, 

 especially of the lungs or air-passages, that the number of respiratory 

 acts increases in quicker proportion than the beats of the pulse ; and, 

 in other 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 mm.) 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 expiratory 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 

 1 mm. during inspiration, and a positive pressure of from 2 to 3 mm. 

 mercury during expiration. 



The instrument used by Hutchinson to gauge the inspiratory and expiratory 

 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 manometer 

 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. During life 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 1 or 2 mm. Hutchinson calculated that the total force to be 



