340 HABITS OF BIRDS. 



CHAPTER XVIII. 



LONGEVITY OF BIRDS. 



IT may be stated as a general principle that animals 

 are long-lived, somewhat in proportion to their 

 size ; and that seems to have some connexion with 

 the rapidity of the circulation of the blood. In the 

 larger animals, such as the elephant, the blood moves 

 slowly ; and in the smaller sorts, such as small 

 birds or mice, the circulation is so rapid that the 

 beats of the pulse can be counted with difficulty or 

 not at all. We are not, however, disposed on this 

 account to infer with the celebrated Boerhaave that 

 the motion of the blood through the arteries and veins 

 tends, by mechanical friction, to destroy the texture of 

 the parts *. We are rather inclined to agree with 

 Baron Haller in referring the apparent effect to the 

 obstructions arising from the minuter vessels being 

 obliterated t- 



Cullen, who partly adopts the opinion of Haller, 

 proceeds upon the three principles that there is a 

 different distribution of the blood in the different 

 periods of life, that the vessels offer a greater resis- 

 tance to the entrance and transmission of the fluids 

 as age advances, and that the excitability gradually 

 decreases. The quantity of the blood is most con- 

 siderable in youth ; and the arteries being then in a 

 state of over-distension, while the system is at the 

 same time more contractile and sensitive, the ten- 



* Epist. ad Ruysch, Lug. Batav. 1722. 

 f Elementa Physiol. xxx. 3. 3. 



