344 CIRCULATION. 



length of time is occupied by the blood in its passage 

 through the entire circuit of both the lesser and greater 

 circulations. 



2. What is the time required for the passage of the entire 

 mass of blood through the heart ? 



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 ani- 

 mals. In 1827, Hering, 1 a German physiologist, performed 

 the experiment of injecting into the jugular vein of a living 

 animal a harmless substance, which could be easily recog- 

 nized by its chemical reactions, and noted the time whicli 

 elapsed before it could be detected in the blood of the vein of 

 the opposite side. This gave the first correct idea of the rapid- 

 ity of the circulation ; for though the older physiologists, such 

 as Haller, Hales, and Keill, had studied the subject, their esti- 

 mates were founded on calculations which had no accurate 

 basis, and gave very different results. The experiment of 

 Hering is often roughly performed as a physiological demon- 

 stration ; and we have thus had frequent occasions to verify, 

 in a general way, its accuracy. If, for example, we expose 

 both jugulars of a dog, inject into one a solution of ferro-cy- 

 anide 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 in twenty or thirty seconds after the injection, 

 the salt has had time to pass from the jugular to the right 

 heart, thence to the lungs and left heart, from this 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 perchlo- 

 ride of iron to the serum, if the specimen be allowed to 

 stand, or a clear extract of the blood may be made by boiling 

 with a little sulphate of soda and filtering, treating the color- 

 less liquid thus obtained with the salt of iron. 



1 MILNE-EDWARDS, Lefons sur la Physiologic, tome iv., p. 362, note. 



