THE CIRCULATION OF THE BLOOD AND LYMPH 123 



easily observed in the jugular veins in man, because of their 

 proximity to the heart. We have already pointed out the signifi- 

 cance of the study of this venous pulse for the analysis of cardiac 

 events (p. 91). 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 

 ' communicated 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 movements (p. 266) are also sometimes spoken of as 

 a venous pulse, but they are produced in an entirely 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 oppo- 

 site 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 appearance 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 

 elimination of the ferrocyanide, and, further, the method was un- 

 suited 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 satisfactory 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. 

 Suppose that the time of the circulation from the jugular to the 

 carotid is required that is, practically the time of the lesser or 

 pulmonary circulation. A small portion of one carotid artery is 



