CHAPTER IX 
EVIDENCE FROM BLOOD TESTS: 
W. B. Scorr 
Here may be conveniently considered the very interesting and 
significant blood tests which have been made in the last fifteen years 
by various physiologists and especially by Dr. George H. F. Nuttall, 
of the University of Cambridge. Though there are several methods 
of making these tests, the “precipitation method’? employed by 
Dr. Nuttall will be quite sufficient for the ends sought in these lec- 
tures. The method and significance of the tests can best be explained 
by taking as an example human blood, which, of course, has been most 
extensively and minutely studied, because of its legal importance as 
well as its scientific interest. Ordinary chemical analysis is unable 
to determine the differences in blood-composition between various 
animals, but that there were important differences had long been 
understood. This was shown by the fact that, in performing the 
operation for the transfusion of blood, it was not practicable to 
substitute animal for human blood, since the former might cause 
serious injury to the patient. 
The precipitation method of making blood tests is as follows: 
Freshly drawn human blood is allowed to coagulate or clot, which it 
will do in a few minutes, if left standing in a dish, and then the serum 
is drained away from the clot. Blood-serum is the watery, almost 
colourless part of the blood, which remains after coagulation. Small 
quantities of this serum are injected, at intervals of one or two days, 
into the veins of a rabbit and cause the formation in the rabbit’s blood 
of an anti-body, analogous to the anti-toxin which is produced in the 
blood of a horse by the injection of diphtheria virus. After the last 
injection the rabbit is allowed to live for several days and is then 
killed and bled, the blood is left until it clots and the serum drained 
off and preserved. The serum obtained thus from a rabbit is called 
“anti-human” serum and is an exceedingly delicate test for human 
blood, not only when the latter is fresh, but also when it is in 
the form of old and dried blood-stains, or even when the blood is 
«From W. B. Scott, The Theory of Evolution (copyright 1917). Used by 
special permission of the publishers, The Macmillan Company. 
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