946 



PHYSIOLOGY 



of Chauveau and Marey. At this point the ventricle is entirely relaxed and 

 the auriculo-ventricular valves open, so allowing the blood to flow freely 

 from the auricle into the ventricle. This negative wave is not always well 

 marked, and is represented only by a series of small undulations in 

 Fig.' 408 A. 



The pressure rises in the left auricle somewhat higher thap in the right 

 auricle. In- the latter case the big veins act as a supplementary reservoir 

 to the auricle, so that in no period of the cardiac cycle need the pressure in 

 the latter chamber rise to any extent. In both the auricular tracings given 

 the heart sounds are apparent as small oscillations in the curve. 



mo 



FIG. 410. Curve of pressures in left auricle of cat. (STRAUB.) 

 I, II, III Ton = 1st, 2nd, and 3rd heart sounds. 



NEGATIVE PRESSURE. When the flow of blood through a tap is suddenly 

 interrupted, the momentum of the fluid tends to make it continue its movement so that 

 a diminution of pressure is produced in the rear of the moving column. In the tracings 

 obtained by means of the older instruments, such as those of Hiirthle, the lever at the 

 end of the ventricular systole descended even below the base line, thus suggesting 

 that for a short period of time there was actually a negative pressure in the ventricles. 

 If the manometer be connected with the heart by a tube provided with a valve allowing 

 the movement of fluid only in one direction, it becomes a maximum or minimum mano- 

 meter according to the direction of the valve. This tube however increases the inertia 

 of the whole system, so that the use of the minimum manometer tends to exaggerate 

 the apparent negative pressure. According to some statements there may be a ne^a t i \ e 

 i 10 of 40 to 00 mm. Hg. in the ventricle at some period during the cardiac cycle. 

 Many explanations were put forward to account for this negative pressure, but i liry arc- 

 all rendered unnecessary by the fact revealed by more perfect graphic methods that at 

 no period in the cardiac cycle is there a negative pressure in the ventricles. Naturally 

 under normal conditions the pressure in the chest outside the heart is negative owing 

 to the elastic retraction of the lungs, and may vary from -3 to -30 mm. Hg., according 

 as it is measured during normal expiration or during forced inspirat ion. This uegat i\ e 

 pressure may In- transmitted to the interior of the heart, but the pressure within the 

 ventricles never falls below the pressure obtaining outside the heart. 



CHANGES IN FORM OF THE HEART 



As the heart walls are perfectly flaccid during diastole, the shape of this 

 organ as a whole will depend upon the position in which the heart is lying 



