LABORATORY EXPERIMENTS 283 



commenced during expiration, is continued, but after a time the diminution 

 is succeeded by a steady rise. The reverse is the case with expiration at 

 first a rise and then a fall. 



As regards the effect of expiration, the capacity of the chest is diminished 

 and the intrathoracic pressure returns to the normal, which is still slightly 

 below the atmospheric pressure. The effect of this on the veins is to in- 

 crease their extravascular and so their intravascular pressure, and to di- 

 minish the flow of blood into the left side of the heart, and with it the general 

 blood pressure. Ordinary expiration does not produce a distinct obstruction 

 to the circulation, as even when the expiration is at an end the intrathoracic 

 pressure is less than the extrathoracic. The effect of violent expiratory 

 efforts, however, does have a distinct action in obstructing the current of 

 blood through the lungs, as seen in the congestion in the exaggerated con- 

 dition of straining, this condition being produced by pressure on the entire 

 group of pulmonary vessels. 



There are other mechanical factors, such, for example, as the effect of 

 the abdominal movements, both in inspiration and in expiration, upon the 

 arteries and veins within the abdomen and of the lower extremities, and 

 the influence of the varying intrathoracic pressure upon the pulmonary 

 vessels, both of which ought to be taken into consideration. As regards 

 the first of these, the effect during inspiration as the cavity of the abdomen 

 is diminished by the descent of the diaphragm should be twofold: on 

 the one hand, blood would be sent upward into the chest by compression 

 of the vena cava. inferior; on the other hand, the passage of bbod down- 

 ward from the chest in the abdominal aorta, and upward in the veins of the 

 lower extremity, would be to a certain extent obstructed. 



LABORATORY EXPERIMENTS IN RESPIRATION. 



1. Respiratory Rate in Man. Count your respirations for from 

 2 to 4 minutes while sitting quietly, and determine the average number per 

 minute. Repeat the counting after standing for 5 minutes, and after brisk 

 exercise. These determinations involve the element of consciousness, under 

 which condition it is difficult for a person to breathe with his normal rate 

 and depth. 



Make a series of determinations of respiratory rates of persons who are 

 sitting quietly but unconscious of your determinations. Count the rates 

 in a number of persons of different ages; where possible, take into considera- 

 tion height, weight, etc. Tabulate the results for a comparison and for 

 future reference. 



2. The Character of Respiratory Movements in Man. A number 

 of instruments have been devised for measuring human respiratory move- 

 ment, many of which measure the change in diameter of the chest in respira- 



