vi FORM AND FUNCTION 103 
the smallest points have been investigated and re- 
investigated. But comparative physiology is a less 
common study. The physiologists, except when wish- 
ing to throw light upon human life, have, as a rule, 
neglected the life of other animals. In default of 
such experiments we must point to the enormous size 
of the air-sacks, far greater than is needed for mere 
breathing, and also to the rate of respiration, which, 
as I have said above, is much greater in a bird, even 
when at rest, than in a man. 
We have not yet done with the machinery by 
which temperature is regulated. There are nerves 
which can cause increased warmth in any organ 
or part of an organ which requires it, and which 
also exercise a general control. If a small artery 
be watched, it will be seen to vary in width without 
any apparent change taking place in the heart’s 
beat. To see this, cut a hole in a thin piece of 
deal, put a frog’s foot over it, and tie the toes 
so that it cannot move them. The frog will suffer 
some discomfort but no actual pain. If now the foot 
be examined under the microscope, the blood will be 
seen circulating, as the skin is quite transparent, and 
the widening and narrowing of the small arteries may 
be made out. The same thing may be seen in a 
small artery in the rabbit’s ear. When little blood is 
wanted ina particular part, the artery which supplies it 
is constricted or tightened. When much is wanted, it is 
dilated. This is effected by the vaso-motor nerves—ze, 
the nerves which act upon the blood vessels—and the 
centre from which they act is believed to be the part 
of the brain which is called the medulla oblongata 
