438 ANIMAL PHYSIOLOGY. 
tant function, and that this was shown by the removal of the 
organ without change in the animal’s economy. 
But to believe that there are no such changes, and to have 
clear proof of it, are two different things. As a matter of fact, 
closer study does show that in some animals there are altera- 
tions in the lymphatic glands and bone-marrow, which organs 
are undoubtedly manufacturers of blood-cells. 
These changes are unquestionably compensatory, and that 
other similar ones corresponding to comparatively unknown 
functions of the spleen have not as yet been discovered is owing 
likely to our failures rather than their real absence. We dwell 
for a moment on this, because it illustrates the danger of the 
sort of reasoning that has been applied in the case of this and 
other organs; and it shows the importance of recognizing the 
force of the general principles of biology, and also the desira- 
bility of refraining from drawing conclusions that are too wide 
for the premises. In every department of physiology it must 
be more and more recognized that what is true of one group 
of animals is not necessarily true of another, or even of other 
individuals, though the differences in the latter case are of 
course usually less marked. We have referred to this be- 
fore, and shall do so again, for it is as yet but too little con- 
sidered. 
Examinations of the spleen, carried out by means of the on- 
cograph, as in the case of the kidney, reveal the following facts: 
1. The spleen undergoes slight changes in volume, correspond- 
ing to the respiratory undulations of blood-pressure, but not, as 
with the kidney, to each heart-beat. 2. The spleen experiences 
rhythmic variations in size, independent of the general blood- 
pressure. It will be borne in mind that the splenic arteries end 
in capillaries, but that some of the arterial blood finds its way 
possibly from the capillaries into the splenic pulp, from which 
it is taken up by veins beginning in this tissue. 
It is highly probable, then, that these movements serve to 
propel the blood that has found its way into the pulp-tissue on- 
ward into the veins; and it is not to be forgotten that among 
large groups of invertebrates, in which capillaries are wanting, 
a not very unlike method of carrying on the general circula- 
tion is found; at the same time, we may suppose that such an 
arrangement of blood-supply and removal would not be un- 
favorable to splenic metabolism. 
There is one fact in the metabolism of the spleen that de- 
serves special notice, though we can not indicate all its bear- 
