RIBS AND STERNUM. 407 



double, the dorsal elements fusing to form bony 

 plates, continuous with those of the vertebrae in 

 front of them. The ventral elements of the 

 transverse processes are rod-like in the case of 

 the first four or five vertebrae, and are regarded 

 as ribs, because they ossify independently of 

 the centra : in the hinder vertebrae, they are 

 smaller. The first pair of these ribs are much 

 stouter than the rest : they are placed in the 

 adult opposite the acetabula, with the walls of 

 which they are connected by bony ridges run- 

 ning across the inner surfaces of the ilia, and 

 separating the fossae for the middle and posterior 

 lobes of the kidneys fiom each other. In the 

 embryo, and in the young chick, these ribs are 

 some distance behind the acetabula, and are no 

 larger than those of the succeeding vertebrae. 

 From the special relation of the bird's sacrum 

 to bipedal locomotion, it exhibits a necessary diver- 

 gence from the sacral vertebrae of other vertebrates, 

 usually two in number. Huxley considered that 

 group (b) are the true sacral vertebrae, on the ground 

 that the spinal nerves associated with them unite 

 to form the sacral plexus. Gegenbaur and others 

 hold that two or more of the vertebrae in group 

 (c) are the true sacral ones, on account of their 

 possessing separately ossified ribs. 

 4 The caudal vertebrae consist of four or five free ones, and 

 a terminal pygostyle. This latter, which supports 

 the rectrices, or large tail feathers, is much larger 

 than the others, and triangular in shape. It is 

 formed by the fusion of five or six vertebrae, which 

 are distinct in the embryo. 



B. The Ribs and Sternum. 

 1 The Ribs. 



a. The cervical ribs. As already noticed, the last two 

 cervical ribs of each side are alone movably 



