96 PHILOSOPHY 'OF ZOOLOGY. 



The sheath, of which we have already spoken, likewise 

 grows in length with the new parts of the feather. It is 

 thickest at first, and falls off in the form of scales ; but, 

 towards the end of the process, it is thin, and adheres 

 closely to the quill, and is that part which requires to be 

 removed by artificial means, in the preparation of quills for 

 writing. 



Feathers appear to consist of nearly the same constituent 

 ingredients as hair. They, perhaps, contain less gelatine. 



They occur only on birds, and characterise the class. 

 They furnish the most obvious marks for the discrimination 

 of species ; and have obtained various names, according 

 to the parts of the body from which they take their rise. 

 These terms will be particularly enumerated when we come 

 to treat of birds. 



3. Horns Horns take their rise in the same situation 

 as hairs or feathers. They may be regarded as hairs ag- 

 glutinated, and forming a hollow cone. The fibrous struc- 

 ture of horn may be perceived in many animals at the bas e? 

 where it unites with the skin. At this part it receives the 

 additions to its growth, the apex of the cone being pushed 

 out in proportion as the increase takes place at the root, 

 and on the inner surface. But horns differ remarkably 

 from hair, in having their central cavity filled by a projection 

 of bone or other solid substance from the body beneath. 



The different markings of the horns, particularly the 

 transverse ridges, are indications of the different layers of 

 growth ; and, in many cases, the number of these ridges 

 corresponds with the years of life. 



The colour of the horn is, in general, distributed through 

 the mass ; sometimes, however, it is collected into bands or 

 threads. It seldom experiences much change during the 

 life of the animal. 



