THE CIRCULATION OF THE BLOOD. 321 



of blood into and through the capillaries; the decrease in volume to 

 the overcoming of the resistance through the arterial recoil. 



The variations in volume may be recorded by enclosing the organ 

 in a rigid glass or metal vessel, which at one point is in com- 

 munication with a recording apparatus, e. g., a tambour with a 

 lever or mercurial manometer with float and pen. The space be- 

 tween the organ and vessel is filled with normal saline, air, or oil. 

 Such an apparatus is known as a plethysmo graph. A well-known 

 form of plethysmograph is that of Mosso (Fig. 153). 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 them is essentially the same. In addition to 

 changes in volume due to the heart's action, most organs undergo 

 additional changes in volume from respiratory and vaso-motor causes. 



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 of blood through the smaller arteries, capillaries, 

 and veins, but 'many of the phenomena connected with it, to which 

 reference has already been made. The structures usually selected 

 for the observation of these phenomena are the interdigital mem- 

 branes, the tongue, the lung, the bladder, and the mesentery of the 

 frog. Though any one of these structures will afford an admirable 

 view of the blood-flow, the mesentery for many reasons is the most 

 satisfactory. For a comparison of the phenomena observed in the 

 cold-blooded animals with those in the warm-blooded animals the 

 omentum of the guinea-pig may be employed. If the frog is the sub- 

 ject of experiment, it should be slightly curarized_and the brain de- 

 stroyed by pithing. The animal is then placed on a small board 

 capable of adjustment to the stage of the microscope. The abdomen 

 is then opened along the side and a loop of intestine withdrawn and 

 placed around a cork ring which surrounds an opening in the side 

 of the frog board. The loop of the intestine should be so placed 

 that it will lie between the observer and the body of the frog. The 

 mesentery thus exposed must be kept moist with normal saline 

 solution. 



When examined with low powers of the microscope, arteries, veins, 

 and capillaries will be found occupying the field of vision. Their 

 general arrangement, their size and connections, can be readily deter- 

 mined. After a few preliminary adjustments a region will be found 

 in which the blood is flowing in opposite directions./The vessel 

 apparently carrying blood away from the observer is an artery; the 

 vessel apparently carrying blood toward the observer is a vein; 



