THE CLASS BIRDS. 307 



fill the interior of the tube, and are called the soul of the 

 feather. 



The young feather is at first enclosed within the sheath of 

 the capsule, which often projects several inches beyond the 

 integuments, and is gradually destroyed. The feather then 

 appears uncovered, and its barbs, at first rolled up, spread out 

 laterally. The extremity of the quill remains embedded in 

 the dermis, but generally may be readily detached, and at a 

 certain period falls, to be replaced by a new feather. This 

 renewal of the feathers is called moulting, and takes place 

 annually, soon after the season for laying the eggs ; but 

 sometimes it occurs twice in the year. At that period the 

 bird loses its voice, and is ill at ease. 



The form of these integumentary appendages varies much; 

 some resemble the spines of the porcupine. In the wing of 

 the cassowary are four or five such ; in others, as in the eagle 

 and raven, the barbs are stiff, and provided with barbules, 

 which being interlocked with those of the adjoining feather, 

 prevents the passage of the air through them. In others, as 

 in the tail and wings of the ostrich, the barbs and barbules are 

 long, soft, silky, and apart. Finally, in others they resemble 

 a kind of down, and this may be seen in certain storks, which 

 are known by the name of marabouts, or pouched adjutants. 

 Their colours vary infinitely, and often surpass in beauty the 

 finest flowers or precious stories. The plumage of the male 

 is generally more brilliant than that of the female ; and it 

 seldom happens that the young bird preserves the same 

 character of plumage throughout life. They often change, 

 for two or three years consecutively, and sometimes the adult 

 has a summer and winter plumage quite distinct. Finally, 

 aquatic birds have their plumage besmeared with an oily fluid, 

 rendering them impenetrable to water, and thus preserving 

 the skin underneath. 



428. The skeleton is composed of nearly the same ele- 

 ments as in mammals, but the form and disposition of many 

 of the bones are different, and, cceteris paribus, their bones 

 are much lighter than those of mammals, being more or less 

 filled with air. 



The head of birds (Fig. 284) is generally small ; in the 

 young bird the cranium is composed of the same number of 

 bones as in mammals, but they unite very early together, and 

 the sutures disappear. The face is in a great measure formed 

 of the jaws, which are much elongated, and being chiefly em- 

 x 2 



