GENERAL RAPIDITY OF THE CIRCULATION. HI 



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 is often 

 roughly performed as a physiological demonstration ; and we have thus had frequent oc- 

 casions to verify, in a general way, its accuracy. If, for example, we expose both jugu- 

 lars in a dog, inject into one a solution of ferrocyanide' of potassium in water, and draw 

 a specimen of blood from the other with as little loss of time as possible, it will be found 

 that, within twenty or thirty seconds after the injection, the salt has had tune to pass 

 from the jugular to the right heart, thence to the lungs and left heart/ and thence 

 through the capillaries of the head and face back to the jugular on the opposite side. 

 Its presence can be determined by the distinct blue color produced on the addition of 

 the perchloride of iron to the serum, if the specimen be allowed to stand, or a clear ex- 

 tract of the blood may be made by boiling with a little sulphate of soda and filtering, 

 treating the colorless liquid thus obtained with the salt of iron. In making the test of 

 the blood-extract or serum, the addition of a drop of nitric acid before the perchloride of 

 iron is added will render the blue reaction much more prompt and distinct if the ferro- 

 cyanide be present. 



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 on which he experimented. Others have taken up these investiga- 

 tions and introduced some modifications in the manipulations. 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 ; but this method is essentially the same as that em- 

 ployed by Hering, and the results obtained by these two observers nearly correspond. 



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. 



