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. 
