232 



GENERAL CHARACTERS. BIRDS.- 



IENERAL CHARACTERS. 



forming the true shaft of the feather, is solid, and 

 composed of a white spongy substance coated with a 

 horny sheath. It tapers gradually to the extremity ; 

 its lower surface exhibits a strong groove, whilst 

 its upper horny, surface is usually rather convex, 

 smooth, and continuous at the base, with the tubular 

 portion of the feather. At the point where the 

 upper horny sheath wrapping round the sides of the 

 stem forms the origin of the hollow quill, there is 

 usually a second small stem, also furnished with webs. 

 This, which is called the plumule, is not found in all 

 birds, and where it does occur, is always confined to 

 the soft feathers forming the clothing of the body ; the 

 quill feathers of the wings and tail being destitute of it. 



The sides of the shaft are occupied by the webs, 

 composed, as already stated, of numerous flattened 

 fibres or barbs. These are inclined towards the apex 

 of the feather. They are usually concave in front and 

 convex behind ; so that they fit together very closely, 

 and their mutual adhesion is provided for by the 

 agency of a series of minute secondary fibres, or 

 barbules, which spring from their margins. These 

 characters are not, however, common to all the barbs 

 even of the same feather. The lower barbs are usually 

 soft and disunited, forming the substance well known 

 as down, and in many of the feathers clothing the 

 body this downy portion constitutes the principal part 

 of the feather, the tip only being formed by a few stiff 

 barbs. The accessory plumule, where it exists, is 

 always of a downy nature, and in a few species of 

 birds the whole plumage shows a tendency to acquire 

 a similar structure. The skins of many birds, espe- 

 cially of the aquatic tribes, are also clothed, beneath 

 the ordinary covering of feathers, with a thick coat 

 of down, which, although evidently analogous in its 

 nature to the true feathers, yet exhibits certain pecu- 

 liarities deserving of special mention. The down 

 consists of a multitude of minute tubes inserted into 

 the skin, from the extremity of each of which there 

 arises a little tuft of soft, disunited filaments. These 

 may evidently be regarded as the barbs of a shaftless 

 feather, and they are furnished on each side with 

 numerous minute fibres representing the barbules. 



In all birds the feathers are changed once or twice 

 in the course of the year, the old feathers falling out 

 by degrees, to be replaced by new ones. This process 

 is called moulting. In it, as 'in the first clothing of 

 the bird with its feathers, these organs are formed in 

 small tubes in the skin, lined by a duplicature of the 

 epidermis. A peculiar fluid secretion is produced at 

 the bottom of each tube; this is soon inclosed in a 

 delicate, conical, horny sheath, with its point directed 

 outwards ; and within this sheath the formation of the 

 feather goes on. As the latter increases in size, the 

 point of its sheath is extended towards the surface of 

 the skin, from which it finally issues, and then, burst- 

 ing, allows the inclosed feather to make its escape. 

 The portion of the formative fluid remaining in the 

 quill after the feather has attained its maturity, is 

 gradually dried up within the cavity, where it forms 

 that peculiar membranous substance which every one 

 must have observed in the quills destined for use 

 na pens. 



In the bats, the only forms of mammals which are 

 endowed with the power of flight, the function is per- 

 formed, as has already been stated, by the agency of 

 broad membranes, which, when they are extended 

 by the elongated fingers, constitute admirable organs 

 for aerial locomotion. In the bats, also, the tail is 

 usually provided with a membrane of greater or less 

 extent, which is of great service to these creatures in 

 directing their course through the air. In birds the 

 same purposes are fulfilled in a very different manner, 

 but still by an extension of the tegumentary appen- 

 dages. The structure of the anterior member in a 

 bird, as already described (see Plate 36), is very differ- 

 ent from that of the same part in a bat. For all 

 practical purposes, the region of the hand may be 

 regarded as reduced to a single finger, so that the 

 limb forms a single series of long joints placed end to 

 end. But the feathers implanted in the skin of these 

 members are of large size and firm texture, and so 

 arranged that when the wing is extended they spread 

 out like the rays of a fan, so as to expose a broad 

 surface to the air ; whilst by the mode in which they 

 mutually assist each other, and their own proper 

 elasticity, their power of resistance is very considerable. 

 At the same time, when the wing is closed, they pack 

 together into a comparatively small compass, and are 

 thus no impediment to their owner in moving about 

 upon the ground or in trees. 



As the number, form, and arrangement of the quill 

 feathers of the wing are of considerable importance in 

 the classification of birds, ornithologists have found it 

 necessary to give them different names, according to 

 the region of the wing upon which they are situated. 

 The longest and strongest, and consequently those 

 which have the most influence upon the power of flight 

 possessed by any bird, are the feathers inserted upon 

 the hand ; these are called primaries. They usually 

 decrease in length from the outer margin of the wing, 

 and in this case the wing is more or less pointed in its 

 outline ; in other cases the longest feather is the fourth 

 or fifth, when the apex of the wing becomes more or 

 less rounded. Their number is usually nine or ten, and 

 sometimes eleven. The name of secondaries is given 

 to the feathers attached to the middle division of the 

 anterior limb, corresponding with the fore-arm of man ; 

 these are shorter and weaker than the primaries, and 

 vary far more in then- number. The terliaries are the 

 feathers attached to the arm. A few small quill fea- 

 thers attached to the rudimentary thumb, form what is 

 called the alula, or spurious wing, and the bases of all 

 the quills are concealed by numerous large but com- 

 paratively soft feathers, forming the wing-coverts, which 

 are distinguished as primary and secondary, according 

 to then- position. 



The quill feathers of the tail, like those of the wings, 

 are long and stiff; they are furnished with muscles, by 

 which they can be spread out to catch the ah- or con- 

 tracted within a small compass, and by the motion of 

 the tail itself they may be turned in various directions. 

 Hence, from their serving in some sort as a rudder for 

 the bird in its aerial course, they have been termed 

 rectrices ; the quills of the wings being also known as 

 remiges, from their being the main instruments of pro- 



