294 Lieut. -Colonel C. B. Heald and Major W. S. Tucker. 



The interpretation put forward here of the recoil peaks must be considered, 

 at present, as purely tentative. 



In fig. 5 it will be seen that the recoils increase and decrease at- regular 

 intervals of time, and the relation of these increases and decreases to respira- 

 tion are well shown in rig. 8. In this figure, the shorter parts of the breathing 

 record correspond to inspiration, and it will be seen that, in agreement with 

 other physiological observations, the maximum recoil occurs just after 

 expiration has commenced (Plate 6). 



The effect on the recoil curve, when the breath is held in deep inspiration 

 and deep expiration, is well shown in fig. 9. 



The recoils, which are so largely increased in inspiration and diminished 

 in expiration, are undoubtedly due to the normal physiological variations 

 in output during respiration. During inspiration, the descent of the 

 diaphragm increases the positive pressure in the abdomen, thereby tending 

 to press blood out of the abdominal veins, and at the same time the negative 

 pressure in the thorax is increased, and greater suction exerted. The heart, 

 therefore, will be better supplied during inspiration with blood than during 

 expiration, and in consequence the output will increase, and enlarged recoils 

 be the result. 



The variation in curves given by apparently healthy individuals may be 

 seen from fig. 10, where the first example demonstrates a simplified type of 

 curve, in which only the primary peaks are recognisable, and from which 

 secondary peaks are practically absent (Plate 7). 



The second example shows the type in which the secondary peaks are so 

 enlarged that they occasionally make recognition of the primary peaks difficult 

 without dividers. 



The third example, taken very shortly after the two upper curves, and 

 without any alteration of the apparatus, is from the standard normal subject 

 •( A. Keading). 



The meaning of these variations is the subject of investigation along two 

 lines, one anatomical, and the other physiological. Small variations in the 

 anatomical position of the axis of the heart, or in the disposition of the 

 great vessels, may, as referred to above, alter the proportion of the total 

 kinetic energy of a contraction acting in the long axis of the body. Or, as 

 Krogh and Lindhard (9) have shown, individuals may have a high or low 

 ■coefficient of oxygen utilisation, with a consequent small or big volume 

 output from the heart, i.e., a small or big recoil curve. 



Variations in Blood Pressure. — As soon as it became evident that the 

 recoil curves were subject to wide normal variations from subject to 

 .subject, a certain number of experiments were carried out to determine 



