Vr KOKM 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 sec 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 in a 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— i.e. 

 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 



