THE CLASS BIRDS, 283 



small septa leave between them. In the corresponding por- 

 tion of the stalk, the bulb is in relation with its inferior 

 surface, and dies; but where the stalk of the feather is 

 tubular, the lamina of horny matter which the bulb pro- 

 duces turns entirely round it, and envelopes it completely. 

 Nevertheless, the bulb, as it fulfils these functions, still 

 dries up and dies, and thus withering successively it forms 

 a series of membranous cones set into each other, which 

 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, arid 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. Their colours vary 

 infinitely, and often surpass in beauty the finest flowers or 

 precious stones. 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 under- 

 neath. 



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

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



