FROG: EXTERNAL FEATURES AND BODY-WALL 27 



system known as the spinal marrow or cord. The muscles 

 of the ventral side are thicker at the ends of the trunk, 

 where they contain bony hoops, the shoulder girdle and 

 hip girdle, which, with the vertebrae between the upper 

 ends of each of them, encircle the body. The ccelom is 

 lined by a smooth membrane, the peritoneum, which is 

 continued over the viscera, so that these are not truly 

 exposed in the body cavity, but hang into it in folds of 

 the peritoneum (Fig. 9). Each fold fits closely over the 

 organ which it suspends, and above the organ the two 

 sides of the fold come together to form a sheet which 

 slings the organ from the body-wall. The largest of these 

 suspensory sheets is that which holds the gut and is known 

 as the mesentery. Between the peritoneum and the muscles 

 of the back is on each side a large dorsal lymph sac, and 

 in each dorsal lymph sac lies one of the pair of kidneys. 

 In the head there is no body cavity, and the backbone is 

 here continued by a large box of bone and cartilage known as 

 the skull, while the spinal cord is prolonged into the skull by 

 the brain. The limbs have neither body cavity nor viscera, 

 and among their muscles lie the bones which support them. 

 The skeleton of the frog is composed chiefly of bone, but 



contains also a good deal of a gristly substance 

 skeleton: known as cartilage. There may be recognised 

 Arrangement, in it an axial part, consisting of the skull, 



backbone, and breastbone, which supports the 

 trunk and head, and an appendicular part, comprising the 

 bones of the limbs and their girdles, which supports the 

 arms and legs and anchors them to the trunk. 



In the backbone there are nine vertebrae and a long 



bone, known as the urostyle, which represents 



several vertebras fused together. The ninth is 

 known as the sacral vertebra, and to it is attached the girdle 

 of the hind limbs. Each vertebra consists of a body or 

 centrum and an arch, the neural arch, placed above the 

 centrum so as to form a ring around the spinal cord. 

 The hollow of each ring is a vertebral foramen, and the rings 

 together form the vertebral canal. The roof of each arch 

 is raised into a low ridge, the neural spine or spinous process, 

 and in every vertebra except the first the arch bears on 

 each side, a little above its junction with the centrum, a 



