50 THE MECHANISM 0E THE CIRCULATION. 



cardium i.s securely fastened. By this means all leakage from the chamber of 

 the instrument is prevented. Finally, the cardiometer is filled with oil, and 

 connected by a tube to a piston recorder. The instrument is experimentally 

 calibrated, so that a certain extent of stroke is known to signify a certain 

 change in volume, and an automatic counter has been devised by Roy and 

 Adami, which records a notch on the tracing at every period when the sum of 

 the upstrokes of the lever corresponds to an output of 100 c.c. of blood from 

 the heart. A simple and adequate form of cardiometer can be made out of 

 a tennis ball. A tube is fixed into the ball and connected with a piston 

 recorder. On the opposite, side to this tube a hole is cut large enough to 

 admit the heart. The pericardium is tied round the ball, and the junction 

 made air-tight by the application of thick vaselin. 1 



The efficiency of the systole can be estimated in yet another way. This 

 is by the simultaneous record of the left auricular and aortic pressures. If 

 the ventricle fail to empty itself, and the residual blood increase, then both 

 the diastolic volume and pressure rise. "While the cardiometer records the 

 increase of volume, a manometer in connection with the left auricle registers 

 the increase in diastolic pressure. 



By one or other of these methods, interesting and important results 

 have been obtained. It is known that, under high pressure, the elastic 

 coefficient of the aorta increases ; the energy of the systole must corre- 

 spondingly increase, in order that the output when the pressure is high 

 may be maintained at the same volume as when the pressure is low. 

 From his researches on the output of the heart, Tigerstedt concludes — 



1. That within certain limits the systolic output is independent of 

 the resistance. 



2. That under favourable conditions a rise of resistance may increase 

 the systolic output. 



3. That with increasing peripheral resistance the systolic output as a 

 rule decreases. 



4. That as the arterial pressure generally increases in spite of the 

 diminished systolic output, the diminution in output per second must 

 be proportionately smaller than the increase in resistance. 2 



The diminution of the output per second is usually shown most 

 strikingly during the rise of arterial tension which is occasioned by 

 asphyxia. In the asphyxial condition the heart muscle rapidly fails 

 and passes into paralytic dilatation ; while the output from the 

 ventricles is opposed, the venous input is enormously assisted by the 

 action of the respiratory spasms. 



A large area of arterioles, mainly the splanchnic, is thrown into 

 constriction, and the blood in the first stage is propelled from the 

 constricting vessels into the veins, 3 while the velocity of flow through 

 the locomotor organs is accelerated owing to a compensatory dilatation 

 of the vessels in these organs, and thus the diastolic filling of the heart 

 is increased. At the same time, under favourable conditions, the heart 

 is accelerated, the systolic output increases, the left auricular pressure 

 falls, and the arterial pressure rises. In the second stage, the vagal 

 centre in the spinal bulb is excited by the high arterial tension, the 

 heart frequency is lessened, and the output diminished. Further, so 

 soon as the arterial pressure reaches a certain point, the heart becomes 

 unequal to the strain of emptying itself against the resistance, the 



1 Hill and Barnard, Brit. Med. Journ., London, 1897. 



- Skandin. Arch. f. Physiol, Leipzig, 1891, Bd. ii. S. 430. 



3 Mall, Arch. f. Physiol., Leipzig, 1892, S. 418. 



