THE CIRCULATION OF THE BLOOD 



375 



recording apparatus, e.g., a tambour with a lever or a piston recorder with 

 float and writing point. The space between the organ and vessel is filled 

 with normal saline, air, or oil. Such an apparatus is known as a plethysmo- 

 graph. Fig. 173. Many forms of this apparatus have been devised in 

 accordance with the character of the organ spleen, kidney, etc. to be 

 investigated, though the principle underlying their construction is essentially 

 the same. In addition to changes in volume due to the heart's action, most 

 organs undergo additional changes in volume from vaso-motor and respira- 

 tory causes. 



Indeed the plethysmographic is the most generally employed method 

 of showing the action of vaso-motor nerves in changing the contraction of the 

 arterioles and hence the outflow of blood. Thus when an organ is enclosed 



FIG. 173. A PLETHYSMOGRAPH FOR THE ARM. 



in a plethysmograph and the arterial contraction increased by either a direct 

 or reflex stimulation of the vaso-motor center there will be a rise in the 

 pressure, a diminution in the outflow of blood and a decrease in the 

 volume of the organ under observation; and on the contrary, if the arteriole 

 contraction is diminished by a direct or reflex inhibition of the vaso-motor 

 center there will be a fall of pressure, an increased outflow of blood and an 

 increase in the volume of the organ. From this it is learned that the func- 

 tional activity of an organ which is attended and conditioned by an increased 

 blood-supply is always associated with an increase in volume. On plethys- 

 mographic records large undulations are frequently observed which are 

 regarded as of respiratory origin. 



THE CAPILLARY CIRCULATION 



In certain regions of the body of many animals it is possible, on account 

 of the delicacy and transparency of the tissues, to observe not only the flow 



