LECTURE I. 27 



power abolished, whilst in general it is lost 

 by degrees. 



The contraction of irritability takes place 

 in some animals in a very slow and gradual 

 manner, and their muscles in general are 

 incapable of sudden contraction. Yet 

 though the action of their muscles is very 

 slow, it is very powerful and very perma- 

 nent. The American sloth supports its 

 weight for a very long time in one attitude, 

 by fixing its claws into the branches of 

 trees ; an act which would speedily weary 

 muscles of an ordinary character. The 

 muscles of the legs of birds that roost, 

 seem to have a similar power of permanent 

 contraction. 



Mr. Carlisle has lately demonstrated a 

 peculiar distribution of the arteries in the 

 limbs of these tardigrade animals, as they 

 are called, and Doctor Macartney has shown 

 that a similar arrangement of vessels exists 

 in the legs of fowls. Such a distribution 

 of the arteries may be subservient with- 



