GENERAL EAPIDITY OF THE CIRCULATION. m 



3. What influence has the number of pulsations of the heart on the general rapidity 

 of the circulation ? 



The first of these questions is the one which has been most satisfactorily answered by 

 experiments on living animals. In 1827, Hering, a German physiologist, performed the 

 experiment of injecting into the jugular vein of a living animal a harmless substance, 

 which could be easily recognized by its chemical reactions, and noted the time which 

 elapsed before it could be detected in the blood of the vein of the opposite side. This 

 gave the first correct idea of the rapidity of the circulation ; for, although the older 

 physiologists had studied the subject, their estimates were founded on calculations which 

 had no accurate basis and gave very varied results. The experiment of Hering, as modi- 

 fied by Bernard, is often roughly performed as a physiological demonstration. The follow- 

 ing is Bernard's method of making this demonstration : In a good-sized dog, expose both 

 external jugular veins as largely as possible. Place a serre-fine upon one jugular at its 

 superior portion, press out the blood below and isolate a portion free from blood, by 

 means of another serre-fine placed on the vein about an inch below the first. Fill this 

 isolated portion of the vein with a strong solution of ferrocyanide of potassium in 

 water, which is injected by means of a hypodermic syringe. The serre-finea are then 

 suddenly removed and the salt immediately begins to circulate in the vessels. The blood 

 is then collected from the opposite jugular at intervals of five seconds. The specimens 

 of blood are then boiled with a little water and the addition of a small quantity of sul- 

 phate of soda, filtered, and the clear fluid is tested with a drop of persulphate of iron, 

 which gives a blue reaction when the ferrocyanide is present. When the experiment is 

 carefully performed, the blue color appears in the extract of blood drawn about ten sec- 

 onds after removal of the serre-fines. In making the test, the addition of a drop of nitric 

 acid before the persulphate of iron is added will render the blue reaction much more 

 prompt and distinct. 



The experiments of Hering were evidently conducted with great care and accuracy. 

 He drew the blood at intervals of five seconds after the commencement of the injection, and 

 thus, by repeated observations, ascertained pretty nearly the rapidity of a circuit of blood 

 in the animals upon which he experimented. Yierordt collected the blood as it flowed, 

 in little vessels fixed on a disk revolving at a known rate, which gave more exactness 

 to the observations. The results obtained by these two observers were nearly identical. 



The length of time occupied by a portion of blood in making a complete circuit of 

 the vascular system, in the human subject, is only to be deduced from observations on 

 the inferior animals ; but, before this application is made, it will be well to examine the 

 objections, if any exist, to the experimental procedure above described. 



The only objection which could be made is, that a saline solution, introduced into the 

 torrent of the circulation, would have a tendency to diffuse itself throughout the whole 

 mass of blood, it might be, with considerable rapidity. This objection to the observa- 

 tions of Hering has been made by Matteucci and is considered by him as fatal to their 

 accuracy. It certainly is an element which should be taken into account ; but, from the 

 definite data which have been obtained concerning the rapidity of the arterial circula- 

 tion and the inferences which are unavoidable with regard to the rapidity of the venous 

 circulation, it would seem that the saline solution must be carried on by the mere rapid- 

 ity of the arterial flow to the capillaries, which are very short, taken up from them, and 

 carried on by the veins, and thus through the entire circuit, before it has had time to 

 diffuse itself to any considerable extent. It is not apparent how this objection can be 

 overcome, for a substance must be used which will mix with the blood, otherwise it 

 could not pass through the capillaries. 



There seems no reason why, with the above restrictions, the results obtained by 

 Hering should not be accepted, and their application be made to the human subject. 



Hering found that the rapidity of the circulation in different animals was in inverse 

 ratio to their size and in direct ratio to the rapidity of the action of the heart. 



