ii2 HOUSE, GARDEN, AND FIELD 



pipe sounding for a long time. They may also enable the 

 bird to aerate its blood comfortably, without taking 

 frequent breaths. But at this point we become aware 

 of a dreadful and irremediable gap in our knowledge of 

 the flying bird. We do not know how frequently it in- 

 spires, and there is no ready way of finding out. A bird 

 at rest inspires very frequently, more frequently than 

 other Vertebrates. But the flying bird ? the skylark ? 

 If he could only tell us how he manages ! 



After the visible mechanism of the skylark's song has 

 been allowed for, questions suggest themselves as to the 

 supply of heart-power and nerve-power, which is implied 

 in full song during hard exercise. Why cannot a man 

 sing when running fast ? We answer, that running makes 

 us " lose our wind." Active exertion quickens the in- 

 spirations, and before long even the quickened rate does 

 not suffice. If the violent exercise is unduly prolonged, 

 we may find ourselves hardly able to breathe at all. But 

 this difficulty of breathing is only an external symptom ; 

 it indicates a disturbance of the bodily functions, which 

 would not be completely provided against by greatly in- 

 creased lung-capacity. Sir Michael Foster has explained 

 to us, in his Rede Lecture on Weariness, what this dis- 

 turbance is. Muscular effort exhausts some part of the 

 living matter of the muscles ; it sets up a greater demand 

 on the blood for oxygen ; the blood draws more oxygen 

 from the lungs, and pours more carbonic acid into them ; 

 strain is put upon the nervous mechanism which regulates 

 the blood-flow. But, whenever muscles contract, other 

 things besides carbonic acid are poured into the blood 

 things which act like poisons upon muscle and nerve. 

 The brain becomes stupefied, the heart distressed, and 

 what we attribute to simple loss of wind is largely, perhaps 

 chiefly, due to a temporary blood-poisoning. There are 

 organs in the body, such as the skin, the kidney, and the 

 liver, whose function it is to remove these poisons, and 

 render them harmless. When the eliminating organs get 



