DISUSE AND ITS EFFECTS. 59 



matrix was in no way irritated, and the sloth lived 

 under admirable sanitary conditions, but spent nearly 

 the whole of its life suspended by its nails. It is well 

 known that the beaks of parrots, when confined in cages, 

 grow very thick and long so as to render it necessary 

 to give them a piece of rough stone whereon they rub the 

 beaks to keep them within reasonable proportions. An 

 unusual beak of this kind is sketched in fig. 30. The 

 bird was found dead in Australia, and an examination 

 of the body did not furnish any evidence leading to the 

 suspicion that it had ever lived in captivity. In this 

 parrot the upper part of the beak measures sixteen 

 centimetres following the curve ; it is difficult to imagine 

 how the bird lived so long. 



Conditions similar to this are often detected in the 

 beaks of partridges, pheasants, peacocks, and fowls. 

 There now remains for consideration atrophy, the result 

 of continuous pressure ; this is of importance because 

 it leads to interesting pathological conditions, and plays 

 also a part of some interest in connection with the 

 normal development of complex animals, but as this 

 subject is beset with technicalities and requires a rather 

 extensive acquaintance with special anatomy, it will not 

 be discussed. 



