282 



AVES. 



sternal keel bespeaks the strength of the ante- 

 rior extremity in the bird. 



Besides the difference of form and deve- 

 lopment of the mesial crest or keel, the ex- 

 tended sternum presents many other varieties 

 in the different orders and families of birds. 

 A zoological arrangement of the class has even 

 been founded on the modifications of this cha- 

 racteristic and important part of the skeleton. 

 In every species the sternum is more or less 



Fig. 129. 



Sternum, coracoida, and clavicles of a Woodpecker. 



quadrilateral, more or less convex outwardly, 

 and each of its margins affords distinctive 

 characters. The anterior margin presents two 

 grooves (b, b, figs. 129, 130) extending along 

 the greater part of either side, and affording a 

 secure articulation to the coracoid bones ; and 

 in many birds it sends forward a process from 

 the middle part where the two grooves meet, 

 as in the Woodpecker and Penguin (e, fig. 

 129). This mesial process we shall term the 

 manubrial process, since it is analogous to that 

 which extends from the manubrium or first 

 sternal bone of the seal, mole, &c. 



The lateral margins are straight and excavated 

 anteriorly, to a greater or less extent, for the 

 lodgement of the sternal ribs. In some birds 

 a process (d,jigs. 129, 130) is given off at 

 each angle of the union of the lateral with the 

 anterior margin- as this process seems to 

 supply the sternal portions of the anterior 

 floating ribs, it may be termed the costal 

 process. 



The posterior margin is most varied in its 

 contour, and is in general interrupted by fis- 

 sures, (f,f,Jigs. 129, 130.) which are always 

 symmetrical in their position, but vary in 

 number and depth, so that this margin is some- 

 times represented by the extremities of three 

 or five long processes. 



In the Diurnal Raptores the sternum is a 

 large elongated parallelogram, convex both in 



the direction of its length and breadth, but 

 especially in the latter sense. The manubrial 

 process is thick, the contour of the keel 

 convex, and its margin extended laterally. 



In the Eagles and Secretary-bird the ster- 

 num is entire, but in the Vultures and Hawks 

 it is pierced on either side by a small round 

 aperture situated near the posterior margin. 

 Ossification sometimes extends along the apo- 

 neurotic membrane stretched over this aperture 

 so as to divide it into two, as has been ob- 

 served in the Buzzard; or so as to obliterate it on 

 one side only, as seen by Meckel in the Kite. 



In the Nocturnal Raptores the sternum is 

 short, convex as in the preceding tribe, but 

 weaker : there is no manubrial process. The 

 keel is less developed, its margin less convex, 

 and not thickened. The posterior margin is 

 concave and presents two fissures, separated 

 by a middle process, except in the common 

 Darn Owl ( Strix flammca ) where it is wanting, 

 and a large but shallow fissure is found in- 

 stead. 



The greater part of the Insessorial Birds are 

 characterized by the following form of sternum. 

 It is large, a little longer than broad, and 

 pinched in, as it were, at the sides, just behind 

 the costal margin. The keel is prominent and 

 convex along its inferior margin ; its anterior 

 margin is slightly excavated, and terminates 

 below in a slightly projecting angle. The ma- 

 nubrial process is compressed, prominent, and 

 curved upwards ; the costal processes are mo- 

 derately developed. The posterior margin pre- 

 sents a single deep fissure OB either side, and 

 a single lateral process, the extremity of which 

 is constantly dilated. The lateral margins are 

 slightly excavated. 



In the CorvidfS the keel is more excavated 

 at its anterior margin ; the manubrial process 

 is stronger, and is bifurcated at the extremity ; 

 the posterior fissures are shallower ; the angular 

 processes directed outwardly and not dilated 

 at the extremity. In the Swallows (Hirun- 

 do) the sternum is large and the keel greatly 

 developed ; there are two posterior fissures, 

 but they are still shallower than in the Crows}; 

 the angular processes are not dilated at the 

 extremities. In the Swifts (Cypselus) the 

 sternum is entire, and corresponds in its pro- 

 portional magnitude with the superior length 

 and power of wing which characterizes this 

 genus. The manubrial process is wanting, but 

 the costal processes are moderately long and 

 pointed. 



In the Humming-birds, which sustain them- 

 selves on the wing during the greater part of 

 the day, and hover above the plant while ex- 

 tracting its juices, the sternum (r, s, Jig. 125,) 

 is still further developed as compared with the 

 body ; it approaches to a triangular form, ex- 

 panding posteriorly, where the margin is entire, 

 and rounded. The depth of the keel exceeds 

 that of the entire breadth of the sternum. The 

 coracoid depressions are deep and approxi- 

 mated ; the manubrial process is small, but 

 evident, and directed upwards ; the costal pro- 

 cesses are also present, but of small size. 



In the Creepers (Certhia) and Hoopoes 



