402 SKELETON OF THE FOWL. 



double y 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 vertebrte, 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 running across the inner 

 surfaces of the Uia, and separating the fossae for 

 the middle and posterior lobes of the kidneys 

 from 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 vertebra. 



It is very doubtful how many of these vertebra?, 

 and which ones, correspond to the sacral vertebrae of 

 other vertebrates, usually two in number. Huxley 

 considers 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 vertebra 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 sup- 

 ports 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 vertebraj, 

 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 arti- 



