28 INTRODUCTION. 



plate, extended over the whole anterior part of the thorax, and 

 even covering more or less of what may be considered the 

 abdomen. In the Golden Eagle, Fig. 4, it is of an elongated 

 quadrilateral form, deeply concave internally, and having a pro- 

 minent ridge or keel along the middle externally, so that, espe- 

 cially with the costal ribs appended, it forcibly reminds one of 

 the form of a ship. It varies greatly in the different tribes of 

 birds, insomuch that it has been proposed as a basis for a 

 methodical arrangement of the class, but in all is more or less 

 four-sided, and externally convex. In the present case, its an- 

 terior margin has a medial prominence, &, named the manubrial 

 process, to which is attached a ligament from the furcula, j ; 

 and two oblique lateral grooves, h, ^, in which are fitted the 

 lower ends of the coracoid bones, ^, h. The lateral edges, c, d^ 

 are nearly straight, and afford attachment to the ribs, ^, g. 

 The posterior margin, f/, e, is nearly straight and transverse. 

 In other species, however, it is variously cut into by sinuses or 

 notches, and consequently exhibits a great diversity of ap- 

 pearance. One object of the large sternum of birds being to 

 afford a basis to the powerful muscles by which the wings are 

 moved, its surface is expanded, and in the median line elevated 

 into a projecting lamina, «,/, named the crest or ridge, by 

 which the muscular masses are kept from blending together. 

 This part, by its greater or less size, indicates a corresponding 

 degree of power in the pectoral muscles, and consequently the 

 strength of flight, although by no means the actual velocity, 

 which depends upon the form of the wing as much as upon 

 the vigour of its muscular apparatus. In the ostrich and other 

 birds of its family, of which the diminutive wings are not used 

 for flying, the crest of the sternum is wanting. This bone has 

 received so extended a development partly to afford a support 

 to the v/ings, the rapid and powerful motions of which ren- 

 dered such an arrangement necessary. These organs come 

 next to be spoken of. 



The Anterior Extremities of birds are modified so as to 

 render them subservient to the aerial progression which these 

 animals have in common with Insects. They are converted 

 into \Aings by having appended to them series of long stiff 



