54 J. C. Douglas — On the Perching of Birds. [Maech, 



when standing : their legs apparently being very readily fatigued by bear- 

 ing the weight of their bodies ; but although most striking in birds, it is also 

 common to mammals. Horses particularly may be observed resting one leg 

 at a time, and man does not as a rule stand equally on both legs, but puts 

 his weight on each leg alternately ; even when sitting on a chair he com- 

 monly rests his legs alternately by hanging one across the other. I think 

 this phenomenon is explained as follows : — Every muscle must have inter- 

 vals of rest, and the muscles, supporting an animal which rests standing, must 

 be rested alternately. To render this possible, the law of muscular exhaustion 

 must in certain cases admit of the load on a muscle being increased, without 

 proportionately hastening exhaustion. A muscle, removed from the body 

 and stimulated to contract, suffers exhaustive loss of irritability, in direct 

 proportion to the rapidity with which the stimuli follow each other, and 

 exhaustion is most readily produced by stimuli following each other so 

 rapidly, as to induce continued contraction, i. e., tetanus ; but the amount 

 of the load, if not too large, does not apparently affect the course of 

 exhaustion, the exhaustion of two muscles bearing different loads being 

 parallel. This being the case, it may be quite possible for an animal 

 to even double the load on one set of muscles, without hastening their 

 exhaustion. The muscles, bearing the additional load, not necessarily 

 being exhausted proportionately sooner than under their own proper load, 

 it is evident a bird resting on one leg does not exhaust it in time shorter 

 in proportion to the increased load ; hence it is enabled to rest its whole 

 system standing, a part at a time, just as completely as it could rest it 

 all at once by laying down. The ease with which a bird stands on one 

 leg is due to the relatively great surface of its base, the length of base, 

 excluding the nails of the toes, reaching in small birds half the bird's 

 height, that is, a man's feet would have to be two and a half to three feet 

 long, to secure a proportional base; the width of a bird's base in front is 

 also relatively great. 



II. — The Perching of Birds. 



The bird, standing on one leg, is only one instance of a very general 

 case, and it rests itself in that attitude by reason of a physiological law 

 governing muscular exhaustion ; but the case of a bird, sleeping on one leg 

 on a perch, has been considered as differing from the case of a bird sleeping 

 on one leg on the ground, and, while it is obvious the bird on the ground 

 really maintains the upright position by balancing itself, precisely as a man 

 does, this explanation has been considered as insufficient in the case of the 

 perching bird. The explanation commonly accepted appears to have origi- 

 nated with Borelli about two hundred years ago, and is as follows : — 



The flexors of the toes pass over the knee and heel in such a manner, 

 that when the leg is bent by the weight of the body, the toes are flexed, the 



