358 PHYSIOLOGY. 



1 millimeter of mercury, during expiration a positive pressure of 

 from 2 to 3 millimeters. 



Forced respirations produce great variations from the above 

 figures. The greatest negative pressure averaged 57 millimeters 

 of mercury during inspiration; the maximum positive pressure dur- 

 ing expiration averaged + 87 millimeters. 



The greater part of the force exerted in deep inspiration is used 

 in overcoming the resistance offered by the elasticity of the lungs, 

 the raising of the weight of the chest, and depressing the abdominal 

 contents. These resisting forces acting during expiration aid the 

 expiratory muscles; from this it follows that the forces concerned 

 in forced expiration are much greater than those of inspiration. 



Expiration is longer and stronger than inspiration,, but the 

 sound of inspiration is longer than that of expiration. 



Fig. 126. Carotid Pressure in Dog. Acceleration of Heart at the 

 Moment of Inspiration is Well Marked. (LANGLOIS.) 



EFFECT OF RESPIRATION ON THE CIRCULATION. 



When a kymographic tracing in an animal is taken there are 

 seen rises and falls in it, due to the acts of respiration. Shortly 

 after the commencement of an inspiration the arterial tension 

 reaches its maximum, and immediately after an expiration it begins 

 to fall, reaching its lowest level after the beginning of the subse- 

 quent inspiration. 



The pulse is more rapid during an inspiration than during an 

 expiration. I shall now inquire into the causes of these two changes : 

 (1) those of blood-pressure, and (2) the increased frequency of the 

 pulse. 



The walls of the air-cells have an elastic force which is greater, 

 the greater the distension. 



This elastic force causes collapse of the lung and exerts a 

 suction-like action on the contents of the chest. This negative 

 pressure becomes greater and greater as the lungs are distended. 





