THE CIRCULATION TIME. 89 



have been devised by Stewart. 1 In these methods no collection or test- 

 ing of the blood is required, and the whole procedure is extremely simple 

 and easy of demonstration. 



In his first method he proceeds as follows : The carotid artery is exposed 

 and placed upon a sheet of insulating material ; non-polarisable electrodes are 

 then applied to the artery, and the portion of the artery between these is 

 inserted as a resistance in one arm of a Wheatstone's bridge. After the re- 

 sistances in the bridge have been balanced, and the galvanometer brought to 

 rest, a small quantity of a solution of common salt is injected into the opposite 

 jugular vein. So soon as the salt reaches the carotid artery the resistance of 

 the blood is altered, the balance of the Wheatstone's bridge is upset, and the 

 galvanometer swings. 



The period between the moment of injection and the commencement of 

 swing can be noted with a stop-watch with great precision. 



In a second and still simpler method, Stewart 2 employs a solution of 

 methylene blue, which he injects into the jugular vein of the rabbit. The 

 carotid artery on the opposite side is exposed, placed upon a sheet of white 

 paper, and illuminated with a strong light. The time is taken with a stop- 

 watch from the moment of injection to the moment when the blue colour is 

 seen to appear in the carotid artery. 



In a rabbit weighing 2 kilos., Stewart found the time of the lesser 

 circulation varied from 2 to 3 seconds, and the jugular to jugular time from 

 5 to 7 seconds. The longest circulation times were found in the kidney, 

 the portal system, and the lower limbs. By section of the vasomotor 

 nerves of any organ the circulation time is lessened in that organ, while 

 it is lengthened by excitation of the vaso-constrictors. 



There is yet another means of arriving at the circulation time, and 

 that is by the injection into a vein of a powerful poison. Thus, in the 

 horse, nitrate of potash 'passes from the jugular vein into the coronary 

 circulation, and arrests the heart in 16 seconds. In the same period strych- 

 nine produces its effect on the spinal cord. In the dog, 12 seconds is 

 sufficient for the manifestation of the action of these poisons ; in the 

 rabbit, 4 to 4*5 seconds ; and in the fowl, 6 to 6*5 seconds. 



The jugular to jugnlar result obtained by Bering's or Stewart's method 

 yields, not the circulation time of the whole of the blood, but the shortest 

 possible time which any particle of blood can travel through the shortest 

 pathway round both the lesser and greater circulation. According to 

 Poiseuille and v. Kries, 3 the maximal or axial velocity is double the 

 mean velocity. A particle of blood swept along in the axial stream, 

 therefore, travels with far greater speed than another particle which 

 rubs along the wet walls of the vessels. Again, the blood that takes a 

 broad, open pathway, through wide or dilated capillaries, will arrive far 

 more speedily at the opposite jugular vein than blood which creeps 

 through tortuous and constricted capillaries. It is impossible to arrive 

 by this method at the circulation time of the whole blood. Part of the 

 blood may travel from the left to the right ventricle, and from the right 

 to the left ventricle, with great rapidity, and part with extreme slowness. 

 The direct observations of Tigerstedt 4 on the output of the left ventricle 

 show that the circulation time of the whole blood is at least five times 



1 Journ. Physiol., Cambridge and London, 1894, vol. xv. p. 1. 



2 "Manual of Physiology," London, 1896. 



3 Beitr. z. Anat. u. Physiol. als Festgabe C. Ludwig, Leipzig, 1887, S. 109. 



4 Skandin, Arch. f. Physiol., Leipzig, 1891, Bd. iii. S. 233 ; "Physiol. des Kreislaufes, 

 1893, S. 467. 



