80 ADVANTAGES IN RESPIRATION. 



may have equal strength in all its parts, have their 

 coats much thicker and firmer than the smaller ones, 

 though we cannot precisely say in what proportion ; 

 neither do we know to what extent the difference of 

 thickness in the coats of the vessels diminishes the 

 action of the air upon the contained fluid. Perhaps 

 the thickness of the coats is directly, and the action 

 of the air inversely, in some such ratio as that of the 

 squares of the diameters of the different vessels ; but 

 this is a mere theoretic guess, undemonstrated, and 

 incapable of demonstration. 



Analogy shows, however, that the advantages which 

 birds derive from this general admission of air to the 

 blood-vessels is very great. The race-horse is quite 

 fatigued with a few miles at his full speed, and so is 

 the greyhound, while the lion himself is in need of 

 repose after a single leap. But birds can maintain 

 their rapid flight during the livelong day, and for 

 hundreds of miles upon the stretch ; and when they 

 do, as is sometimes the case, drop down in agitation 

 or in exhaustion, the former seems generally to be the 

 eftect of fear, and the latter of muscular fatigue, for 

 they do not pant as the mammalia do when they 

 have over-exerted themselves. 



But the advantage in the aeration of the blood is 

 not the only one which birds derive from this pecu- 

 liarity of their structure. The same air which exerts its 

 renovating influence upon the blood, supports all the 

 more delicate structures which it reaches and sur- 

 rounds, as a cushion of the most perfect softness and 

 elasticity ; so that, by the most rapid motion, and the 

 most violent twitches which the body receives in the 

 changes and turnings of that motion, there can be no 

 concussion of the parts more immediately necessary 

 for the life of the bird. 



