BIRD. 



471 



sternum. This keel is convex on the under side 

 and concave on the front, the angle pointed, and 

 surmounted by a small bony process outside. The 

 lateral processes are inclined forward and upward, 

 and there are six ribs between them and the posterior 

 lateral ones. Coracoids short, but well formed, and 

 moderately well set forgiving- firmness to the shoulder 

 joint. The clavicle is a long semi-oval, rather slender, 

 but. without any process on the under side at the union 

 of the two branches; but there is a small tubercular 

 process on the upper side. The scapulars are very 

 long, and a little bent downwards, and thin. 



This sternal apparatus indicates considerable power 

 of flight, though, from its narrowness, the stroke of 

 the wing must be quick and rather feeble. The 

 central posterior process gives a firm support to the 

 middle of the under part of the body ; and the 

 lateral ones, from their great length and flexibility, 

 support the flanks, and at the same time enable the 

 body to accommodate itself to those obstacles among 

 whieli, from their habits, the birds have to find their 

 way. The narrowness of the sternum in proportion 

 to its length, and the accordant general shape of the 

 bird, also conduce lo the same purpose, and the 

 straightness of the sternum enables it better to resist 

 pressure in the direction of its length than the curved 

 sterna of other birds, though these support much 

 more powerfully the flying motion of the wing. 



Many of the birds which have the sternum of this 

 form are swift and smooth runners, and all, or at least 

 most of them, run with the head advanced nearly in 

 a line with the axis of the body, which is carried 

 horizontally. The head, too, is email and pointed, 

 and the shoulders are narrow and tapering. This 

 form of the body enables the birds to make their way 

 among the tall herbage of humid places, and especially 

 the banks of brooks and streams, and the reedy 

 margins of pools and shallow lakes in rich and flat 

 countries, where they seek the greater part of their 

 food. Many, if not all of them, can at times enter 

 the water, and, when they are immersed to a sufficient 

 depth, they use their wings t'ormolion in that element. 

 The narrow sternum, and the consequently short and 

 rapid motion of the wings, are much better adapted 

 for action in the water than wings which take longer 

 strokes, and are more powerful and efficient, in the 

 air. This can be readily understood when it is borne 

 in mind that the raising or recovering of the wing is 

 a much more laborious operation in water than in air, 

 and that the power requisite for working long wings, 

 almost to the full circle, as is done by air birds, would 

 in water be greater than any ordinary structure of a 

 bird could be supposed to possess. Besides, when 

 wintrs are used under water, they must keep time 

 with the feet, and hence they are never moved far 

 from the horizontal plane of the body, but strike 

 short and quick, and move most forcibly downwards 

 or upwards, according as the course is ascending or 

 descending. 



The natural transition here is not to the running 

 and wading 1 birds, which report, at least iu most of the 

 species, to the banks, shores, and other humid places, 

 which are not so tangled and rank with vegetation 

 as the haunts of the tribe now considered. These 

 carry the type in the general form of the body and 

 the sternal apparatus, though modified by the habit 

 and haunt, from t'.ie n-rebes, which unite considerable 

 powers of flight with swimming and diving, through 

 the divers properly so called, to the penguins, which 



are incapable of flight ; but those species which use 

 the wings chiefly in the water as a sort of fins have 

 the sternal apparatus, of course, different from those 

 that use them in the air. 



Sterna of the Divers. There is so much diversity 

 of habit, as well as of sternal structure, among these, 

 that it is impossible to select any single species as 

 an average type. We shall, therefore, give three 

 instances which correspond in some measure to the 

 two extremes and the mean the grebes, the true 

 divers, and the penguins. 



1. Sterna of the Grebes. The sternum of these 

 birds is intermediate, in its general form, between 

 that of the coots, as the aquatic extreme of the pre- 

 ceding group, and that of some of the diving ducks, 

 or rather those of the eiders and scoters, which are 

 intermediate between the swimmers and divers of the 

 duck tribe. The sternum is short and broad, and 

 much broader in the rear than the front; but the 

 breadth in that part does not consist so much of 

 continuous bone as of two posterior lateral processes, 

 which are divided from the central part of the bone 

 by notches of considerable depth. These processes 

 are much stronger, as well as shorter, tlian those of 

 the birds in the preceding family. They are arched, 

 rising upward at their divisions, and recurving again 

 toward the middle portion, though not joining it. 

 The keel is well developed, though its height is not 

 equal to the breadth of one side of the sternum. It 

 is convex on the under side, and concave in front, with 

 the angle sharp, but without any production of bone. 

 The margin is triangular and cartilaginous. The 

 coracoids are of moderate length, rounded on their 

 anterior sides, and flattened and much enlarged in the 

 rear. They are strong, and well set for giving firmness 

 to the shoulder-joint, as they form a considerable angle 

 with each other, and also with the general line of the 

 sternum ; so that the strain which they exert when 

 the wings are moved is directed toward the point of 

 greatest resistance in that bone, which is, of course, 

 also the mean centre of the surfaces to which the 

 muscles which move the wings are attached. The 

 clavicle is long, forming a regtdar semi-ellipse, 

 attached by a small tubercle at the middle to the 

 angle of the keel, flattened laterally, and forming, from 

 the head of the coracoid to the keel, a perfect arch, 

 with its convex side to the front, and the terminations 

 of its branches bear against the heads of the scapulars, 

 the greater part of which is slender, and slightly bent 

 downwards. The general shape may be seen in the 

 following two figures of the sternal apparatus of the 

 eared grebe half the size of nature. 



The breadth of this sternum, the convexity of its 

 under side, the form and size of its keel, the strength 

 and setting of the coracoids, and the opening and 

 curved form of the clavicle, all indicate considerable 

 action of the wings. Its stoutness and firmness in the 

 posterior part also indicate considerable action of the 

 legs, not of the tarsi merely, but also of the tibia% 

 and not in the vertical plane, as in the common 

 operation of walking, but in a direction contrary, or 

 at all events oblique, to that plane. A sternum which 

 best suits for a walking motion is that which has the 

 posterior angles taken off, so that the whole has a 

 lozenge shape, as in poultry and pigeons ; and one 

 which answers best for the alternate foot motion of 

 swimming on the surface of the water is of a punt 

 or boat shape, which bears up the body equally. But 

 the sternum of the grebe does not answer to any of 



