THE ANATOMY OF A BIBD 29 



them, except the middle two, connected with a system of ramified 

 air-spaces which, as ahready said, permeates the body generally, 

 lying among the viscera, between the muscles below the skin, and 

 deep into the actual interior of the bones. But though it is difficult 

 to see this by a dissection, it is easy enough to prove it by inflating 

 them. If a syringe is passed down the windpipe and tied carefully 

 into it, so that no air can escape at the sides, and air is blown down 

 the tube, the passage of the air into the skin and other parts can 

 be followed ; if a bone be cut across, the air can be noticed to issue 

 from the cut surface ; and if the experiment be varied by using a 

 coloured fluid instead of air — which is pumped in by a syringe— 

 the fluid can be seen to ooze from the end of any bone or muscle 

 that has been cut across. A bird, therefore, when it takes in a deep 

 breath, not only supplies its lungs with fresh air, but fills its whole 

 body with the superfluous air. It has been proved that a bird can 

 continue to breathe if it be held under water, and only the end of a 

 broken limb allowed above the surface ; for, as aU the spaces of air 

 are in communication with the lungs, they (the lungs) can obviously 

 be as conveniently filled from one end as from the other. "When 

 you are bathing, and take a very deep breath as you are swimming, 

 you can detect a sensible increase in the buoyancy of the body ; in 

 a bird, of course, the difference is enormous, after the sacs are filled, 

 from a condition of comparative emptiness. The way in which a 

 bird breathes is different from the way in which a human being 

 breathes. There is, of course, the essential resemblance that is shown 

 between all animals that have definite organs which are set apart 

 for respiration : the feathery gills of the marine worms, the closely 

 Bet branchiae of the fish, the lungs of the bird and of the mammal, are 

 aU constructed upon one plan, so far as essentials are concerned. In 

 all of them blood-vessels are brought into close relation, though not 

 into actual contact, with water or air containing oxygen. The blood- 

 vessels are separated from the water or air by the thin membranes 

 of the lungs or gills, through which the oxygen can pass in to the 

 blood, and the carbonic acid and effete gases can pass out ; it is this 

 exchange which is the essential act of respiration. We cannot, 

 however, in this book pretend to go into general matters of this kind, 

 which would take us too far from the subject at hand ; but anyone 

 who would pursue this fm-ther can consult Professor Huxley's ' Ele- 

 mentary Physiology,' or any other elementary text-book upon physio- 

 logy. When a mammal — a human being, for example — breathes 

 certain muscles are called into play. If a person is watched, it will 



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