AVES. 



349 



Peculiar Secret ions. The unctuous fluid 

 with which Birds lubricate their feathers is 

 secreted by a gland which is situated above the 

 coccyx or uropygium. This gland consists of 

 two lateral moieties conjoined. As might be 

 expected, it is largest in the birds svhich frequent 

 the water. In the Swan it is an inch and a 

 half in length, and has a central cavity, which 

 serves as a receptacle for the accumulated secre- 

 tion; but this cavity has not been observed in 

 other species. Each lateral portion is of a 

 pyriform shape, and they are conjoined at the 

 apices, which are directed backwards and are 

 perforated by numerous orifices. The longitu- 

 dinal central cavities also present internally nu- 

 merous angular openings, in which there are 

 still smaller orifices. The surrounding glandu- 

 lar substance consists of close-set almost paral- 

 lel straight tubules, and is not irregularly cellular. 

 The tubules extend to the superficies of the 

 gland, without ramifying or intercommunicating, 

 and preserve an equable diameter to their blind 

 extremities. The tubules are longest at the 

 thickest part of the gland, and become shorter 

 and shorter towards the apex. 



Tegumentary system. This is composed, as 

 in Mammalia and Reptilia, of the corium or 

 derm, epiderm, and its appendages, and an 

 intermediate layer of unhardened epiderm with 

 colouring matter, called rete mucosum. 



The corium, or true skin, is very thin, as in 

 the cold-blooded Ovipara. It adheres io the 

 subcutaneous muscles by cellular tissue, which 

 is frequently the seat of accumulation of dense 

 yellow fat ; and it is moved by muscles which 

 at the same time raise and ruffle the plumage 

 which it supports. 



The rete mucosum rarely contains any co- 

 louring matter where the feathers grow ; at 

 this part the skin is of a pale, greyish colour, 

 or pink, from the colour of the blood which 

 circulates in it. But in the naked parts of 

 the integument, as the cire, the lore, the 

 comb, the wattles, the naked parts of the head 

 and neck in some birds, and the tarsi and 

 toes, the rete mucosum frequently glows with 

 the richest crimson, orange, purple, green, 

 black, and a variety of other tints, of which 

 the planches calorics and the different zoological 

 monographs of families of birds afford nu- 

 merous examples. 



The epidermis is in some places continued 

 as a simple layer over the corium, following 

 its wrinkles and folds, as around the naked necks 

 of some Vultures. It is moulded upon the 

 bony mandibles to form the beak, and in some 

 birds adheres to osseous protuberances on the 

 cranium, where it forms a species of horn ; and 

 it is remarkable that these instances occur chiefly 

 in those orders of birds, the Cursores and 

 Rasores, which are most analogous to the Ru- 

 minantia among quadrupeds: the Cassowary 

 and Helmeted Curassow are examples. The 

 cuticle is sometimes developed into spines or 

 spurs, as upon the wing of the Secretary-bird, 



t Cassowary, the Apteryx, and the Palamedea; 

 md upon the tarsi of the Gallinaceous Birds. 

 The claws which sheath the ungueal phalanges 

 >f the feet assume various forms adapted to 



the habits and manner of life of the different 

 orders. A remarkable artificial form is given 

 to the claw of the middle toe in certain birds ; 

 the inner edge being produced and divided 

 into small parallel processes like the close-set 

 teeth of a comb (Jig. 132.) These teeth are not 

 reflected or recurved, as they might be expected 

 to be, if they had been intended to serve as 

 holders of a slippery prey, but are either placed 

 at right angles to the claw or are inclined to- 

 wards its point. The Common Barn-Owl (Strix 

 flammed), the Goat-sucker genus ( Caprimul- 

 gus), the Heron and Bittern kind (Ardeidtg, 

 Vig.), afford examples of this structure ; and 

 as each species of bird appears to be infested 

 by its peculiar louse (Nirmus), the solution 

 of the final intention of so singular a con- 

 trivance, which is limited to so few species, 

 and these of such different habits, may yet 

 be afforded by the entomologist. At least 

 it would be worth while to examine the pa- 

 rasitic animals of the species so provided, with 

 the view of determining whether they pos- 

 sessed superior powers of adhesion which 

 might require the application of a comb in the 

 birds infested by them.* 



With respect to the scales 'which defend 

 the naked parts of the legs of birds, they do 

 not differ from those of Reptiles. Their form 

 and disposition, as has been already observed, 

 have afforded distinctive characters to the zoo- 

 logist. In most of the Raptores, the Psitta- 

 cidte, the Rasores, the Grallatores, and the 

 Natatorcs, the scales are polygonal, small, 

 and .disposed in a reticulate form ; the birds 

 so characterized formed the Retipedes of Sco- 

 poli. In the rest of the class the tarsi are 

 covered anteriorly with unequal semi-annular 

 scales, ending on each side in a longitudinal 

 furrow, and these birds were termed the * Scu- 

 tipedes.'-^ 



The four classes of vertebrate animals have 

 each their characteristic external covering : the 

 cold-blooded Ovipara are naked, or their ex- 

 ternal surface is defended only by hard scales 

 or plates ( squama and scuta); but the warm- 

 blooded classes require to be invested by an 

 integument better adapted to maintain the high 

 degree of temperature peculiar to them : hence 

 quadrupeds are clothed with fur and hair, and 

 birds with down and feathers. 



Feathers are the most complicated of -all 

 the modifications of the epidermic system, 

 and are quite peculiar to the class of birds. 

 The eloquent Paley well observes that " every 



* Mr. Swainson objects to>. the theory which 

 ascribes to the serrated claw the function of freeing 

 the plumage from vermin, because its presence is 

 partial in the class of Birds. ' To suppose," says 

 he, t( that nature has given to one or two families 

 of birds the exclusive power of freeing themselves 

 from an enemy which in like manner infests all 

 birds, is preposterous." The assertion that the 

 different species of Nirmi infest all birds in like 

 manner is much easier than the proof. 



fin one section of the Tyranni, Cuv. the scutae 

 surround the tarsi as complete rings. Where the 

 carneous parts of the muscles are continued low 

 down upon the legs, as in the Owls, a covering of 

 feathers is co-extended to preserve their tempera- 

 ture. 



