24 AN INTRODUCTION TO ZOOLOGY 



The skeleton of the frog consists of two different tissues : a softer, 

 more elastic one, cartilage ; and a harder, firmer one, bone. In the 

 tadpole, and even in the young frog, the entire skeleton is formed of 

 cartilage, but as it grows up, although some cartilage persists, the 

 greater part of it is changed. It is replaced by bone, and, in addition, 

 other bones are formed where no cartilages existed previously. 

 These latter are formed by bone tissue being laid down in a membrane, 

 and hence are termed membrane bones, to distinguish them from 

 cartilage bones, which were preceded by cartilage. This is an 

 important distinction that must be borne in mind when considering 

 the vertebrate skeleton. 



The skeleton forms a framework, giving support, rigidity and 

 shape to the whole body. It serves in the main for the attachment 

 of muscles by providing a series of solid structures, which at the 

 same time act as levers and form joints. Although principally 

 concerned with muscular activity, indeed, mainly with locomotion, 

 the skeleton also serves as a protection for the underlying organs 

 in some parts, e,g. the sternum protects the heart, and the neural 

 canal and skull protect the spinal cord and the brain. 



Like the whole animal the skeleton may be divided into an axial 

 portion, consisting of the vertebral column and the skull, and an 

 appendicular portion, composed of the limbs and the girdles, by means 

 of which they are attached to the body. 



Vertebral Column. 



The backbone consists of nine short separate parts or 

 vertebrae, and a much longer posterior one, the urostyle. All the 

 vertebrae, except the first one and the last two/are more or less similar, 

 and any one can be taken as typical. A single vertebra forms an 

 entire bony ring surrounding the neural canal, with a thickened 

 ventral part, the centrum, upon which is supported, the hoop-shaped 

 neural arch. The front end of the centrum is concave and the 

 hinder end convex, a condition which we describe as pro-ccelus. 

 The two adjacent ends of succeeding vertebrae fit into one another, 

 and so form a shallow ball and socket joint. To ensure ease of 

 motion they are capped with smooth articular cartilage, and between 

 them is a fluid-filled space, the synovia! cavity. This is surrounded 

 at its periphery by a tough tissue, the intervertebral capsular liga- 

 ment. In the preparation of the skeleton both cavity and ligament 

 are destroyed. 



The neural arch on each side is composed of a vertical pedicle 

 and a more expanded horizontal lamina, forming the roof of the 

 neural canal. In the mid-dorsal line it bears a short backwardly- 



