PLETHYSMOGRAPHY. 



189 



The development of the heart and the large blood-vessels is responsible for 

 certain differences in the distribution of the blood in children and in adults. From 

 childhood to puberty the heart is relatively small and the vessels are relatively 

 large. After puberty, on the contrary, the heart is large and the arteries are 

 comparatively small. Accordingly, the arterial blood-pressure in the greater cir- 

 culation must be lower in a child than in an adult. The pulmonary artery is 

 relatively large in childhood, the aorta relatively small; after the onset of puberty 

 both arteries are approximately of the same 'size. Hence, it follows that the 

 blood-pressure in the pulmonary vessels of the child must be relatively higher 

 than in the adult. 



PLETHYSMOGRAPHY. 



The plethysmograph is an instrument employed to determine and register the 

 amount of blo'od in an extremity and its variations. It is a perfected apparatus, 

 modeled after the " box-sphygmometer" described by Chelius in 1850 (Fig. 41). It 

 consists of a long container (G), designed for the reception of an entire extremity. 

 The opening around the introduced part is made air-tight by means of rubber, and 

 the interior of the vessel is tilled with water. In the lateral wall of the receptacle 

 is a communicating tube, which also is filled with water to a certain level. As 

 each pulse-beat causes an enlargement of the extremity as a result of the increased 

 flow of arterial blood, the water in the tube will indicate the magnitude of this 

 positive variation in the quantity of blood, which will be transmitted to the drum 

 (T), covered with an elastic membrane, and with which is connected a writing 

 lever moving in a horizontal direction. 



The cylinder G may also be filled with air. v. Kries connects the tube with 

 a gas-burner instead of with the registering drum (T) , so that the variations in 

 the size of the arm are reproduced in the flame, the flickerings of which may be 

 photographed. 



FIG. 73. Mosso's Plethysmograph: F, communicating flask, by elevation of the level of which the hydrostatic 

 " pressure may be increased; T, the inscribing apparatus. 



Individual organs (spleen, kidney) may be enclosed in a box-like apparatus 

 in a similar manner for the purpose of observing fluctuations in their size: onco- 

 graph. 



The fluctuations of the plethysmograph permit recognition of the following 

 phenomena: 



i. Pulsatory fluctuations in volume. As the venous current in the resting 

 extremity may be regarded as uniform, any rise in the volume-curve must indicate 

 a greater velocity in the movement of the arterial blood-current toward the periph- 

 ery, and the reverse. The curves registered by this apparatus represent volume- 

 pulsations and resemble a dromographic curve" (Fig. 69, III). A rise in the limb 

 of the curve indicates a greater flow of arterial blood, while a fall indicates a 

 diminution in the flow. If the level of the curve remains the same, it is to be 

 inferred that the arterial inflow of blood is equal to the venous outflow. 



At first sight the plethysmographic tracing (volume-curve, current-pulse) 

 presents a great similarity to" the sphygmographic tracing (pressure-pulse), espe- 

 cially from the fact that" both exhibit the dicrotic elevation. More careful ex- 

 amination, however, reveals several differences: In the plethysmographic tracing 

 (current-pulse) the curve descends to a much lower level after the primary apex. 



