be regarded as a reaction of
infection rather than of immunization as ordinarily understood, for it
is found that the blood serum of patients suffering from typhoid, Malta
fever, cholera, and many other bacterial diseases, agglutinates the
corresponding organisms. This fact has come to be of great importance in
diagnosis.

The precipitin test depends on a somewhat analogous reaction. If the
serum of an animal be injected repeatedly into another animal of
different species, a "precipitin" appears in the serum of the animal
treated, which causes a precipitate when added to the serum of the first
animal. The special importance of this fact is that it can be utilized
as a method of distinguishing between human blood and that of animals,
which is often of importance in medical jurisprudence.

In this summary the facts adduced are practically all biological, and
are due to the extraordinary activity with which the study of
bacteriology (q.v.) has been pursued in recent years. The chemistry of
the blood has not hitherto been found to give information of clinical or
diagnostic importance, and nothing need here be added to what is said
above on the physiology of the blood. Enough has been said, however, to
show the extraordinary complexity of the apparently simple blood serum.

The methods at present employed in examining the blood clinically are:
the enumeration of the red and white corpuscles per cubic millimetre;
the estimation of the percentage of haemoglobin and of the specific
gravity of the blood; the microscopic examination of freshly-drawn blood
and of blood films made upon cover-glasses, fixed and stained. In
special cases the alkalinity and the rapidity of coagulation may be
ascertained, or the blood may be examined bacteriologically. We have no
universally accepted means of estimating, during life, the total amount
of blood in the body, though the method of J.S. Haldane and J. Lorrain
Smith, in which the total oxygen capacity of the blood is estimated, and
its total volume worked out from that datum, has seemed to promise
important results (_Journ. of Physiol_. vol. xxv. p. 331, 1900). After
death the amount of blood sometimes seems to be increased, and
sometimes, as in "pernicious anaemia," it is certainly diminished. But
the high counts of red corpuscles which are occasionally reported as
evidence of plethora or increase of the total blood are really only
indications of concentration of the fluid except in certa