MECHANICS OF THE CIRCULATION IN THE VESSELS 135 



of cardiac events (p. 100). A jugular venous pulse of a perfectly 

 different origin is seen in cases of incompetence of the tricuspid 

 valve. Here the chief elevation is synchronous with the ventricular 

 systole, and is caused by the regurgitation of blood from the right 

 ventricle through the auricle into the veins. The so-called ' com- 

 municated venous pulse ' is simply due to the proximity of some 

 large artery, especially when enclosed in a common sheath, whose 

 pulsations are directly transmitted to the vein. The changes of 

 pressure in the great veins accompanying the respiratory move- 

 ments (p. 290) are also sometimes spoken of as a venous pulse, but 

 they are produced in a different way namely, by the rhythmical 

 alteration in the intrathoracic pressure, which alternately favours 

 and hinders the venous return to the heart. 



The Circulation-Time. Hering was the first who attempted to 

 measure the time required by the blood, or by a blood-corpuscle, to 

 complete the circuit of the vascular system. He injected a solution of 

 potassium ferrocyanide into a vein (generally the jugular), and collected 

 blood at intervals from the corresponding vein of the opposite side. 

 After the blood had clotted, he tested for the ferrocyanide by addition 

 of ferric chloride to the serum. The first of the samples that gave the 

 Prussian blue reaction corresponded to the time when the injected salt 

 had just completed the circulation. This method was improved by 

 Vierordt, who arranged a number of cups on a revolving disc below the 

 vein from which the blood was to be taken. In these cups samples of 

 the blood were received, and the rate of rotation of the disc being known, 

 it was possible to measure the interval between the injection and appear- 

 ance of the salt with considerable accuracy. Hermann made a further 

 advance by allowing the blood to play upon a revolving drum covered 

 with paper soaked in ferric chloride, and by using the less poisonous 

 sodium ferrocyanide for injection. 



Even as thus modified, the method laboured under serious defects. 

 It was not possible to make more than a single observation on one 

 animal, at least without allowing a considerable interval for the elimina- 

 tion of the ferrocyanide, and, further, the method was unsuited for the 

 estimation of the circulation-time in individual organs. In both of 

 these respects the more recently introduced electrical method presents 

 considerable advantages; for by its aid we can not only obtain satis- 

 factory measurements of the circulation-time in such organs as the 

 lungs, liver, kidney, etc., but we can repeat them an indefinite number 

 of times on the same animal. 



A cannula, connected with a burette (or a Mariotte's bottle, or a 

 syringe), containing a solution of sodium chloride (usually a 1*5 to 

 2 per cent, solution), is tied into a vessel say the jugular vein. Sup- 

 pose that the time of the circulation from the jugular to the carotid is 

 required that is, practically the time of the lesser or pulmonary circu- 

 lation. A small portion of one carotid artery is isolated, and laid on 

 a pair of hook-shaped platinum electrodes,* covered, except on the 

 concave side of the hook, with a layer of insulating varnish. To 

 further secure insulation, a bit of very thin sheet -indiarubber is slipped 

 between the artery and the tissues. By means of the electrodes the 



* The electrodes can easily be made by beating out one end of a piece of 

 thick platinum wire to a breadth of 5 or 6 mm., and then bending the flattened 

 part into a hook, or by bending pieces of stout platinum foil, 



