RECORD OF THE HEART BEAT 101 



the downward stroke due to its relaxation is more marked than the 

 upward stroke of the contraction, which was partly masked by the 

 ventricular relaxation. The rise from b to c is due to the contraction 

 of the auricles. It is at first rapid and then gradually slows. The 

 descent from c to d is due to relaxation of the auricles, and from d to 

 e is the first part of the ventricular contraction. The processes occur- 

 ring just before and at d are in two directions. First there is the 

 relaxation of the auricles which would allow the lever to descend, and 

 secondly there is the ventricular contraction which causes the lever to 

 ascend. The actual movement of the lever is the algebraic sum of 

 these two movements, and the upward movement of the ventricle soon 

 exceeds in amount the fall due to the auricular relaxation. The part 

 of the curve from e to / is especially interesting. It also is due to 

 the contraction of the ventricle, so that this contraction occurs in two 

 stages. The meaning becomes clear if we cut the two aortse when we 

 find that the second stage, e to /, immediately disappears. It is also 

 only present in tracings produced from hearts which receive a full 

 blood supply. It proves that the ventricle at first is able to empty 

 itself comparatively easily, and therefore shortens rapidly. At the 

 point c, however, the pressure inside is suddenly raised, and the con- 

 traction becomes slower. The last part of the blood is ejected with 

 greater difficulty than the first. This receives a further support from 

 a study of the arrangement of the blood vessels springing from the 

 aortse. The first and main part of the blood is sent into the pul- 

 monary and aortic arches, which offer a relatively low resistance. The 

 pressure here is thus rapidly raised until it reaches such a height that 

 it is able to overcome the resistance of the third arches, and blood then 

 passes into the carotids. 



In II and III the characters of the beat are somewhat different. 

 In III the heart was beating slower than in II, and it is seen that the 

 main increase in rapidity occurs during the systole. The part from a to 

 b is the auricular systole ; from c to d the ventricular. The second 

 stage of the ventricular systole is only faintly indicated in a few beats, 

 and in most it is quite absent. In a quite bloodless heart this second 

 stage is entirely absent (see, for instance, fig. 100, p. 123). 



7. EXCISE THE HEART. Remove the frog from the myo- 

 graph plate and take away the hook from the ventricle. Count the 

 number of beats in a minute. Cut through the two aortee close to the 

 bulbus if they have not already been severed. Lift up the ventricle 

 by the apex and cut through the pericardium, so as to expose the 

 sinus venosus ; cut through the inferior cava and the two superior cavae 

 as they come into view, and thus remove the whole heart as in fig. 82. 



