270 THE CIRCULATION OF THE BLOOD 



necting with the artery as possible. When all the apparatus is arranged 

 and the animal anesthetized, it should be tied firmly to the animal-holder. 

 Let one experimenter attend strictly and at all times to anesthetizing the animal; 

 recovery to light anesthesia must not occur. Let the operator quickly expose 

 about 3 cm. of the carotid artery (or the femoral artery if circulation time 

 is also to be tested on this animal), by making an incision through the skin 

 of the neck 5 cm. long, and dissecting down between the muscles. Sepa- 

 rate the carotid from the adherent vagus nerve by tearing the connective 

 tissue with the scalpel handle, freeing the vessel for about 2 to 3 cm. of its 

 length. Lay two loose ligatures of linen thread around the vessel, place 

 a small bulldog forceps on the exposed artery nearest the heart, and ligate 

 the end nearest the head with one of the ligatures. Lift up the inter- 

 vening artery with strong forceps and make a V-shaped cut near the liga- 

 ture pointing the cut toward the heart, let it extend about half-way across 

 the artery. Introduce a cannula toward the heart, and tie it firmly with 

 the second ligature. Connect the cannula with the rubber tubing to 

 the vertical glass tube. 



When all is ready remove the bulldog forceps from the artery. The 

 blood will flow freely from the artery into the tube, rising by rapid spurts 

 until the pressure from the column of liquid is just equal to that inside 

 the artery itself. If an anticoagulant-like powdered potassium oxalate 

 is first introduced into the vertical tube, probably clotting at the cannula 

 will be delayed for some minutes. The mounting of the blood into the 

 empty tube makes, indeed, a most striking demonstration. 



An accurate measure of the height of the top of the column above the 

 level of the cannula at the artery represents the arterial blood-pressure in 

 terms of blood. The specific gravity of blood is 1.056; of mercury, 13.6. 

 Record the pressure you obtain in terms of blood and of mercury. Note 

 also the variations in pressure and account for the rhythm of each. 

 There will be a general variation of pressure, depending upon the degree 

 of anesthesia. If anesthesia is light and muscular movements happen, 

 there will be an increase in the blood-pressure. If the anesthesia is 

 heavy, then the blood-pressure falls. These points of variation should 

 be marked and recorded at once in note-books. Make full notes of all 

 accessory facts which would aid in explaining the variations in blood- 

 pressure, such as size of the animal, rate of respiration, rate of heart-beat, 

 the variations in anesthesia, the presence of the reflexes, etc., etc. 



Chloroform the animal to kill it, and note the change in blood-pressure 

 during the process, but first do experiment 15. 



15. The Circulation Time. The circulation time is most satisfactorily 

 determined in the laboratory by introducing a saline solution of methylene 

 blue into the jugular vein on one side. Note the exact time with a stop- 

 watch until the color appears in the carotid artery, and in the jugular 

 vein of the opposite side. 



