236 THE CIRCULATION OF THE BLOOD 



13. The Circulation Time. The circulation time is most satis- 

 factorily determined in the laboratory by introducing a saline solution of 

 methylene blue into the jugular vein on one side. Note directly the time 

 with a stop-watch until the color appears in the jugular artery and the jugular 

 vein of the opposite side. 



Anesthetize a cat or dog with a chloroform-ether mixture, tie it on the 

 animal-holder and, when the eye reflexes are lost, expose the jugular vein on 

 the right side, the carotid artery and the jugular vein on the left. Fill a 

 2-cm. hypodermic syringe with i per cent methylene blue in physiological 

 saline, insert the needle into the right jugular vein, pointing it toward the heart. 

 Lift the left carotid artery and place under it a strip of moist white paper 

 2 cm. wide; prepare the left jugular vein in the same way. Place the animal 

 so that these vessels are lighted to the best advantage. At a given moment 

 inject the contents of the hypodermic syringe, noting the time with a stop- 

 watch. Observe the color of the left carotid and the left jugular, respec- 

 tively, very carefully, and take the time when the first appearance of the 

 methylene blue is noted. The color will appear first in the artery, second 

 in the vein. The difference in time between the moment of injection and 

 the moment of color in the artery represents, with a slight correction, the 

 circulation time of the pulmonary or lesser circulation. The time from the 

 injection until the color in the jugular vein represents the total time of circu- 

 lation. 



Stewart has made these determinations even more correctly by the elec- 

 trical-resistance method. He injected 10 per cent salt solution and deter- 

 mined the variation in resistance by a galvanometer. If the galvanometer 

 is available, then check the above determinations by the electrical method, 

 arranging the apparatus under the direction of an instructor. 



14. The Blood -Pressure Model. An artificial model of the cir- 

 culatory apparatus, which illustrates all mechanical parts involved, has 

 been arranged by Porter, figure 217. Other forms, which show these as well, 

 are usually available or can be easily constructed. The model should have 

 the following possibilities: A pump, which permits of rhythmic action at a 

 varying rate and varying force; a resistance to the outflow liquid, which can 

 be increased or decreased; and an elastic set of vessels into which the pump 

 discharges. 



If Porter's schema is used, determine the following points: The pressure 

 in terms of mercury in the arterial and venous limbs of the apparatus when the 

 pump makes a rate of 72 per minute; the influence on these two pressures 

 when the rate is increased, when it is decreased ; the effect on these pres- 

 sures when the peripheral resistance is great, when it is low. If a sphygmo- 

 graph is available, take a tracing of the pulse in the elastic tube representing 

 the arterial side of the schema. 



If an ordinary bulb syringe and simple apparatus is used, then deter- 



