216 ANIMAL PHYSIOLOGY. 
of the cells in reflex action; and, in the fourth, we have an in- 
stance of inhibition. The latter may also be rendered clear by 
the case of a horse and its rider. The horse is controlled by the 
rider, who may be compared to the center, through the reins 
answering to the nerves, though it is not possible for either rider 
or reins to originate the movements of the animal, except as 
they may be stimuli, which latter are only effective when there 
are suitable conditions—when, in fact, the subject is irritable 
in the physiological sense. 
THE CIRCULATION OF THE BLOOD. 
Every tissue, every cell, requiring constant nourishment, 
some means must necessarily have been provided for the con- 
veyance of the blood to all parts of the organism. We now 
enter upon the consideration of the mechanisms by which this 
is accomplished and the method of their regulation. 
Let us consider possible mechanisms, and then inquire into 
their defects and the extent to which they are found embodied 
in nature. 
That there must be a central pump of some kind is evident. 
Assume that it is one-chambered, and with an outflow-pipe 
which is continued to form aninflow-pipe. This might be pro- 
vided with valves at the openings, by which energy would be 
saved by the prevention of regurgitation. In such a system 
things must go from bad to worse, as the tissues, by constantly 
using up the prepared material of the blood, and adding to it 
their waste products, would effect their own gradual starvation 
and poisoning. 
It might be conceived, however, that waste at all events was 
got rid of by the blood being conducted through some elimi- 
nating organs; and assume that one such at least is set aside 
for respiratory work. If the blood in its course anywhere 
passed through such organs, the end would be attained in some 
degree; but if the division of labor were considerable, we should 
‘suppose that, gaseous interchange being so very important as 
we have been led to see from the study of the chapters on gen- 
eral biology, and on muscle, organs to accomplish this work 
might receive the blood in due‘course and return it to the cen- 
tral pump in a condition eminently fit from a respiratory point 
of view. 
Such, however, would necessarily be associated with a more 
