694 EVOLUTION OF ANIMALS Part V 



cover the second, third, and fourth vertebrae. Joined to these are the coracoids 

 and clavicles; the latter known in man as the collarbones, in chickens as the 

 wishbone. The upper bone (humerus) of the foreleg fits into a cavity where 

 the coracoid and clavicle come together. The pelvic girdle is formed by the 

 long innominate bones each composed of three pieces, the ilium, pubis, and 

 ischium, that are joined together to form the sockets for the femurs of the hind 

 legs. The spread of the anterior end of the girdle accounts for the hump back 

 of the resting frog. The solitary sacral vertebra is the only anchor for the frog's 

 pelvic girdle and appended hind legs. This arrangement provides for the re- 

 bound needed in the rear of a jumping animal. It does not support weight like 

 the human pelvic girdle with its attachment to the fused sacral vertebrae (Fig. 

 34.12). 



The arrangement and in general the number of the bones of the fore and 

 hindlimbs of the frog are similar to those of human limbs. In the forelimb, 

 however, the radius and ulna are permanently crossed. A frog cannot turn its 

 forefoot, "palm up." 



Muscles. The main kinds of muscular tissues and arrangements of muscles 

 are discussed in Chapter 10. 



Body Cavities. Body cavities are bounded by the body wall, by mesentery 

 or other membranes, or by combinations of these. In the frog, the main ones 

 are the pericardial cavity containing the heart and the pleuroperitoneal cavity. 

 The latter is called pleura from the membrane that covers the lungs and lines 

 the spaces surrounding them, combined with peritoneum, the lining of the ab- 

 dominal walls (Fig. 34.13). The peritoneum is a transparent, moist, shimmer- 

 ing membrane that continues, as part of the mesenteries, around the stomach, 

 intestines, and other abdominal organs and forms a partial capsule about each 

 kidney (Fig. 34.14). It is so thin that tissue fluid filters through it and, becom- 

 ing the coelomic fluid, keeps the surfaces of the organs wet and slippery. Folds 

 of the intestine and lobes of the liver slide upon one another. The internal 

 organs of a breathing animal are slightly but continually moved. 



Food and Digestion. With its tongue, a frog jerks small animals into its 

 mouth and throws them down its throat (Fig. 34.15). It clutches larger ones 

 by its maxillary teeth and by vomerine teeth on the roof of the mouth. The 

 movement of material through one or another part of the alimentary canal is 

 aided by contraction of muscles (peristalsis), lubrication by mucus, and down- 

 ward lashing of cilia. The pharynx begins where the mouth region narrows 

 backward at the level of the internal nares, and ends by gradually merging 

 into the esophagus. Its lining bears constantly lashing cilia. If powdered chalk 

 or small bits of cork are scattered on the roof of the pharynx they will instantly 

 begin moving into the esophagus. The latter is short, capable of great exten- 

 sion, with strong muscular walls that contract peristaltically and urge the food 

 over the slippery lining. 



