22 COMPARATIVE ANATOMY 



named because it lies along the main longitudinal axis of the 

 body, consists of the skull and the vertebral column. We 

 will first consider the vertebral column. It is made up of ten 

 pieces, nine being peculiarly shaped rings of bone called 

 vertebrae, the tenth being a long rod-shaped piece called the 

 urostyle ; it is the representative of a number of fused 

 vertebrae. 



A typical vertebra (the fourth or fifth in the frog will serve 

 as a type) consists of a short cylinder of bone called the body 

 or centrum, on the top of which is placed a flat bony arch 

 in such wise that the centrum below and the arch above 

 enclose a space through which in life the spinal marrow passes ; 

 hence it is called the neural arch. The bodies of the vertebrae 

 are jointed together, and the arches are also connected by slid- 

 ing joints called zygapophyses (Greek (Vyov, a yoke; aTro^o-is, 

 a process), and so the nine arches enclose a canal, the neural 

 canal, or canal of the spinal cord. Examining the vertebra 

 more closely it will be seen that its posterior end is a rounded 

 knob covered with smooth white cartilage, while its anterior 

 end presents a concavity lined with cartilage. The bodies of 

 the vertebrae are, in fact, connected by ball and socket joints, 

 and the ball is on the hinder, the socket on the anterior, face 

 of each vertebra, excepting the first, eighth, and ninth. The 

 centrum of the eighth vertebra is concave at both ends ; that of 

 the ninth is convex anteriorly, and posteriorly it has two small 

 rounded processes for articulation with the urostyle. The first 

 vertebra will be more particularly described further on. A 

 vertebral centrum which is concave in front and convex behind 

 is called procoelous ; one which is, like the eighth vertebra, 

 concave both anteriorly and posteriorly, is called amphicoelous ; 

 and one which is convex in front and concave behind (not 

 represented in the frog) is called opisthocoelous (TT/OO, in 

 front of ; oVtcrtfe, behind ; a/xc/u, on both sides ; /coiAo?, hollow). 

 If the centrum of a vertebra is sawn across the middle it will be 

 found to contain a central cavity filled with a peculiar tissue. 

 This is the remnant of the notochord or chorda dorsalis, the 

 primitive skeletal rod which formed the backbone of the 

 embryo, and has been replaced by bone. A notochord is the 

 first part of the skeleton to be formed in the embryos of all 

 Vertebrates. In some it is persistent throughout life ; in others 

 it is surrounded by, or replaced by, cartilage or bone, but traces, 



