SEC. Ill EXTERIOR PARTS OF BIRDS 155 



retrorsum, backward ; here used in the sense of in an opposite 

 direction from the lay of the general plumage ; but they should 

 properly be called antrorse, i.e. forward). The nostrils, whether 

 culminal or lateral, are, like the eyes and ears, always two in 

 number, though they may be united in one tube, as in the petrels. 



The Gape. — It only remains to consider what results from the 

 relations of the two mandibles to each other. When the bill is 

 opened, there is a cleft or fissure between them ; this is the gape or 

 rictus (Lat. rictus, mouth in the act of grinning). But while thus 

 really meaning the open space between the mandibles, it is generally 

 used to signify the line of their closure. Commissure (Lat. committere, to 

 put or join together) means the point where the gape ends behind, 

 that is, the angle of the mouth, angulus oris, where the apposed 

 edges of the mandibles join each other ; but, as in the last case, it 

 is loosely applied to the whole line of closure, from true commissure 

 to tip of the bill. So we say, " commissure straight," or " commis- 

 sure 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. 2 6, j) be the true cutting edges of either mandible 

 from tip to opposite base of bill proper; rictus (Fig. 26, g) be their 

 edges thence to the point commissure (Fig. 26, h) where they join 

 when the bill is open; the LINE commissure (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. Angulation of the commissure is a 

 distinctive character of most finches and buntings — Fringillidce. It 

 is well shown in the hawfinch, Coccothraustes vulgaris. 



II. THE WINGS. 



Definition. — Pair of anterior or pectoral limbs organised for 

 flight by means of dermal outgrowths. 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 

 loons, cormorants, dippers. Wanting in no recent birds, but im- 

 perfect in a few, as all Batitce ; greatly reduced in the emeu, casso- 

 wary, and kiwi-kiwi ; also in the moas (Dinornis); in the Cretaceous 

 Hesperornis only the rudimentary humerus is known. To under- 

 stand 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 



