A REPRESlsNTATIVE VERTEBRATE. FROG 



333 



into the large intestine. The digestive canal, 

 the urinary bladder, and the reproductive 

 ducts open into an enlarged cavity called the 

 cloaca. Waste products and reproductive 

 cells pass from the cloaca to the outside 

 through the cloacal opening. In the frog 

 the cloacal opening is often, but incorrectly, 

 called the anus. An anus is the posterior 

 opening of only the digestive system. The 

 cloacal opening is the common posterior 

 aperture through which the products of the 

 intestine, kidneys, and reproductive organs 

 pass to the outside of the body. The intes- 

 tine is held in place by a dorsal fold of the 

 peritoneum. The layers of cells that make up 

 the intestmal wall (Fig. 215) consist first of 

 a ver}' thin outer coat of peritoneum. Be- 

 neath this is a layer of longitudinal muscle 

 fibers, then a thicker layer of circular muscle 

 fibers; next comes a connective tissue layer, 

 the submucosa, containing numerous blood 

 vessels, separated more or less from the in- 

 nermost layer, the mucosa, by a thin layer of 

 fibrous connective tissue, the lamina (tu- 

 nica ) propria. The mucosal epithelium con- 

 sists of two types of cylindrical cells forming 

 a single layer: (1) absorptive cells, which 



are narrow, and (2) goblet cells, which 

 produce a slippery mucus. The mucosa is 

 thrown into many folds but no true villi nor 

 definite glands and crypts are present as in 

 higher vertebrates. 



Digestion in vertebrates is fully treated in 

 Chapter 32. 



Respiratory system 

 and respiration 



The primary functions of the respiratory 

 system are to provide oxygen to the tissues 

 and to get rid of excess carbon dioxide. 



Two kinds of respiration may be recog- 

 nized: (1) external respiration, whereby 

 oxygen in the air enters the body and is 

 transported to the cells, and carbon dioxide 

 is carried away from the cells to the outside 

 of the body. There are two successive phases 

 to this process: (a) breathing, which brings 

 oxygen and blood together in the lungs, and 

 (b) transportation of oxygen and carbon 

 dioxide between the lungs and cells. (2) In- 

 ternal respiration, during which the blood 

 supplies oxygen to, and takes carbon dioxide 

 from the cells of the body. Oxygen in the 



External nares 



Nasal cavity 



Esophagus 



Internal 

 nares 



Tongue 



Mouth cavity Glottis Lung 



Figure 216. Respiratory movements of the frog. In diagram at left, the external nares are 

 open and air enters the mouth cavity. In diagram at right, the external nares are closed, the floor 

 of the mouth cavity is raised, and air is forced into the lungs. Labels have been omitted from 

 the very short larynx and bronchus. Arrows show pathway of air to lungs. 



