110 CIRCULATION 



has been investigated by the use of nutrient solutions of 

 definitely known composition. The results obtained are 

 briefly as follows : Nutrient fluids for the heart must be alkaline 

 in reaction. Sodium carbonate is the alkali generally used. 

 It has no specific action, but neutralizes the carbon dioxide 

 and acids formed by the activity of the heart muscle. Sodium 

 chloride must be present of a strength isotonic with the blood 

 of the animal. Some calcium salt to prevent the diffusion of 

 calcium out of the muscle fibers is essential to continued con- 

 tractions. Calcium salts tend to produce prolonged tonic 

 contractions, and this effect is neutralized by the addition 

 of potassium salts. When. calcium salts are removed from a 

 solution by the addition of oxalate compounds, the heart 

 ceases to beat, but spontaneous contractions return when 

 calcium is again added. A well-known nutrient solution is 

 Ringer's, which is a mixture of 100 c.c. of a 0.6 per cent, sodium 

 chloride solution saturated w r ith tribasic calcium phosphate and 

 2 c.c. of a 1 per cent, solution of potassium chloride. Oxygen 

 is essential to the prolonged activity of the heart. Carbon 

 dioxide is injurious when present in large quantities. A heart 

 poisoned with the latter substance shows an irregular series 

 of contractions. It has not been satisfactorily demonstrated 

 that organic substances are immediately necessary to the 

 rhythmic activity of the heart. 



Arteries, Capillaries, and Veins. The continuously high 

 pressure that exists in the aorta causes the blood to move 

 to points of lower pressure, and it is thus kept in constant 

 movement from the arteries through the capillaries to the 

 veins, and so back to the heart, where, by the action of this 

 organ it is again transferred into the artery and put under 

 high pressure. Blood pressure is usually measured by an instru- 

 ment called a mercury manometer. It consists of a U-shaped 

 glass tube, the bend of which is filled with mercury. One 

 limb of the tube, filled with an anticoagulation fluid, is put 

 in connection with the bloodvessel of the animal, while the 

 surface of the mercury in the other limb carries a small float 

 to which is attached a delicate pen that bears against a hori- 

 zontally moving surface. Such an arrangement is a kymograph. 

 Variations of the blood pressure within the vessel are trans- 

 mitted through the fluids to the mercury, which moves up 



