VERTEBRAL COLUMN 373 



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 vertebras 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 which they are connected by 

 bony ridges running across the inner surfaces of 

 the ilia, 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 aceta- 

 bula, and are no larger than those of the 

 succeeding vertebrte. 



It is very doubtful how many of these vertebrae, 

 and which ones, correspond to the sacral vertebrte of 

 other vertebrates, usually two in number. Huxley 

 considers that group (b) are the true sacral vertebra, 

 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 sup- 

 ports the rectrices, or tail feathers, is much larger 

 than the others, and triangular in shape. It is 

 formed by the fusion of a number of vertebrae, which 

 are distinct in the embryo. 



B. The Bibs and Sterniuu. 

 1. The ribs. 



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

 cervical ribs of each side are alone movably arti- 

 culated, the remainder being fused with the trans- 

 verse processes of the corresponding vertebrae. 



