EXTERNAL PARTS OF BIRDS. — THE WINGS. Ill 



i. e., forward). Nostrils, whether cuhninal or lateral, are, like eyes and ears, always two in 

 nuinher, though they may be united in one tube, as in Petrels. 



The Gape. — It remains to consider what results from relations of mandibles to each 

 other. When a bill is opened, there is a cleft or fissure between upper and under mandibles ; 

 this is the grape or rictus (hat. rictus, mouth in the act of grinning). Though thus really 

 meaning the open space between maudibles, gape generally signifies the line of their closure. 

 Commissure (Lat. committere, to put or join together) properly means the point where the 

 gape ends behind — that is, angle of mouth, angulus oris, where apposed edges of mandibles 

 join each other; but, like gape, it is extended to the whole line of closure, from commissural 

 point to tip of bill. So we say, "commissure straight," or "commissure curved"; also, 

 "commissural edge "of either mandible (equivalent to " tomial edge"), in distinction from 

 culmen or gonys. But it would be well to have more precision in this matter. Let, then, 

 tomia (fig. 26, j) be the true cutting edges of either mandible from tip to base of bill proper,: 

 let rictus (fig. 26, g) be their edges thence to the commissural point (fig. 26, h) where they 

 join Avhen the bill is open ; commissural line (fig. 26, /) to include both when the bill is closed. 

 The gape is straight, when rictus and tomia are both straight and lie in the same line; curved, 

 sinuate, when they lie in the same curved or waved line ; angulated, when they are straight, 

 or nearly so, but do not lie in the same line, and therefore meet at an angle. (An important 

 distinction : see under family FringilUdce in the Synopsis.) 



The "Egg Tooth." — Finally, it is to be observed that unhatched birds are provided 

 with a tool for working their way into the world by chipping the eggshell. This interesting 

 instrument is a small sharp knob or boss at the tip of the upper mandible, such as also exists 

 in some reptiles ; it may readily be observed in a newly hatched chick of domestic fowl. It 

 consists of a deposit of hard calcareous matter in the middle layers of epidermis, not connected 

 with the underlying bone, but breaking through the epidermal layers to come in contact with 

 tlie eggshell that is to be chipped at one point and thus cracked open. Soon after hatching, the 

 calcareous substance of this curious little drill is cast ofi", and the layers of epidermis through 

 which the point of the drill projected cease to be distinguishable from the rest of the horny 

 covering of the bill. 



II. THE WINGS. 



Definition. — Pair of anterior or pectoral Kmbs organized for flight by means of epidennal 

 outgrowths (feathers). Used for this purpose by birds in general ; but by Ostriches and their 

 allies only as outriggers to aid running; by Penguins as fins for swimming under water; used 

 also in the latter capacity by some birds that fly well, as Divers, Cormorants, Dippers. Want- 

 ing in no recent birds, but imperfect in all Batitee, among which the wings are greatly reduced 

 in the Emeu, Cassowary, and Apteryx, while in Moas {Dinornithid(c), as in the Cretaceous 

 Hesperornis, only a rudimentary humerus is known. To understand their structure we must 

 notice particularly 



The Bony Framework (figs. 27, 28, 29). — The skeleton of a bird's wing is built upon 

 a plan common to the fore or pectoral limb of most vertebrates, so that its bones and joints may 

 readily be compared and identified with those of any lizard or mammal, including man. But 

 the member is highly specialized ; being fitted for accomplishing flight, not only by develop- 

 ment of feathers, but also by modifications in the bones themselves. The axes of the bones 

 have a special direction with reference to each other and to the axes of the body ; the move- 

 Tnents of the joints are peculiar in some respects ; and the end of the wing, from the wrist out- 

 vanl. is peculiarly constructed, by loss of some of the digits that five-fingered animals possess. 



