THE VERTEBRATE BODY 



309 



does not occur because the ends of the bones are padded with cartilage at 

 the joints and there is a lubricating liquid, the synovial fluid, that further 

 reduces friction. If a person receives an injury to the knee joint this 

 fluid may leak out and cause a swelling under the skin called ''water on 

 the knee." The bones must also be held in place at a joint and there are 

 tough membranes, called ligaments, that run from one bone to the other 

 to hold them together. 



There are also many immovable joints where the bones come together 

 in such a way that there is little or no movement possible. There is no 

 joint cavity and no synovial fluid. Such joints are found between the 



Photo by Winchester 



Fig. 22.1. Leopard frog. Note the tympanic membrane just posterior and ventral 

 to the eye and the prominent sacral hump on the back. 



bones of the skull which meet along jagged, saw-toothed lines called 

 sutures. Such joints allow growth of the bones during development of 

 the brain. They also allow some movement of the bones of the skull 

 during childbirth. In mature adults many of these sutures become fused 

 together and growth in size of the skull ceases. 



The skeleton of the frog may be divided into the following parts : 



I. The axial skeleton. 



A. The skull. Composed of numerous bones, all of which are 

 immovably fused together except the lower jaw which has 

 a movable joint enabling the frog to open its mouth. 



1. Brain case. 



2. Auditory capsules. 



