850 Proceedings of the Boyal Society 
blood continue to live, for a certain time at least, in the body of 
their host, was to be assumed from some of the very earliest experi- 
ments on transfusion made by Dr Lower in 1666, in which dogs 
continued in good health, after all the blood in their bodies had 
been replaced by that obtained from other dogs. 
The first observer, however, to make direct observations on the 
actual duration of the life of such transplanted or transfused blood 
corpuscles was Panum in 1863. 
His method was crude and inaccurate. He judged of the number 
of corpuscles present in any quantity of blood by the difference in 
specific gravity between the blood serum, and the blood defibrinated. 
In this way, after withdrawing blood from a dog and replacing it 
with defibrinated blood, he was enabled to show that two days 
later the number of blood corpuscles remained almost unaltered, 
and that five days later the majority of them still remained within 
the circulation. 
It was naturally impossible by this rough method to determine 
more closely the further fate of the transplanted corpuscles. This 
could only be done by actual enumeration of the corpuscles before 
and after the injection, a method of investigation at that time un- 
known ; but even since its introduction, the results obtained have 
not been so definite as d priori might have been expected. They 
serve, however, to throw some light on the subject. 
There are two ways in which, by means of transfusion, information 
as to the duration of the life of red blood corpuscles may be ob- 
tained, viz., either by transfusion of blood into an animal without 
foregoing depletion, or by transfusion of blood after previous with- 
drawal of some of the- 'animal’s own blood. 
In the latter case, the difficulty is to determine afterwards what 
proportion of the blood corpuscles found present belong to the 
animal, and what proportion is derived from the transfused blood ; 
since by the withdrawal of blood the standard of comparison, — 
the number of corpuscles originally present, — has been lost. The 
difficulty is one which from its very nature it is quite impossible 
for us to overcome. 
It might be thought that in the former method, viz., transfusion 
without any foregoing depletion, we have at hand a ready means 
of ascertaining the duration of life of the red blood corpuscles, since 
