428 ANIMAL PHYSIOLOGY. 
explain that filtration is not to be understood in its ordinary 
laboratory acceptation, but that the glomeruli discriminate as 
to what they allow to pass, yet they in no way explain how 
this isdone. They make the whole process depend on blood- 
pressure, and attribute little special action to the flat epithe- 
lium of the Malpighian capsules. 
Though we can not admit the full force of Heidenhain’s ex- 
periments as he interprets them, we still believe that his views 
are most in harmony with the general laws of biology and the 
special facts of renal secretion. Recently, after a repetition of 
Nussbatim’s experiments, and the institution of others, it has 
been rendered clear that the mechanical theory of the work of 
the kidney can not hold, even of the glomeruli, which are 
shown to be, as we should have expected, true secreting organs. 
Now, there can be no doubt that blood-pressure is a most im- 
portant determining condition here as in other secreting pro- 
cesses, in the mammal at all events; but whether of itself or 
because of the influence it has on the rapidity of blood-fiow, it 
is difficult to determine; or rather whether solely to the latter, 
for that the constant supply of. fresh blood is a regular con- 
dition of normal secretion there can be no doubt, Further, it 
seems probable that blood-pressure has more to do with the 
secretion of water than any other constituent of urine. But 
we maintain that it should be called a genuine secretion, and 
that nothing is gained by using the term “ filtration ”—on the 
contrary, that it is misleading, and tends to divert attention 
from the real though often hidden nature of vital processes, 
The facts of disease and the evidence of therapeutics, we think, 
all favor such a view of the work of the kidneys. 
Nerves having an influence over the secretion of urine simi- 
jar to those acting on the digestive glands have not yet been 
determined. The powerful influence of emotion, especially in 
those of unstable nervous system, over the secretion of urine 
shows that there must be nervous channels through which the 
nerve-centers act on the kidneys; though whether the results 
are not wholly dependent upon vaso-motor effects may be con- 
sidered as an open question by many. We think such a view 
improbable in the highest degree. The most recent investiga- 
tions would seem to show that the vaso-motor fibers run in the 
dorsal nerves, especially'the eleventh, twelfth, and thirteenth, 
and that of these the vaso-constrictors are the best developed. 
Pathological—_-'™When the kidneys are excised, the ureters 
ligatured, or when the former are so diseased as to be inca- 
