178 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Digestive Cavities (black) 

 Adequate digestive area developed within limited size of body 



Moutti 



Tops of cells reocti 

 into sac 



B 



Branches and out 

 pouches from sac 



Crop 

 Gizzard 



Locol enlargements and 

 out pouches from tube 



Stomoch 



D 



Local enlargement and tube 

 double the length of body 



intestine in 

 watch-spring coil 



Stomach 



Local enlargements, coils 

 and branches (caeca) 



Fig. 11.6. Diagrams of digestive cavities (black). They are examples of rel- 

 atively large capacity and area for secretory and absorptive cells contained within 

 limited body size: A, Hydra, a sac into which the ends of secretory cells are 

 extended; B, planarian, a sac with outpouchings that reach out through the body 

 and partly take the place of circulating blood; C, earthworm, local enlargements, 

 a common device for greater capacity; £>, snail, lengthening and doubling-back of 

 the digestive tube, mainly in mollusks; E, tadpole, the intestine, a watchspring coil; 

 F, bird, enlargements (crop, stomach, gizzard), coils and branches (caeca). The 

 foregoing are the chief patterns of the alimentary tract in multicellular animals; 

 no account has been taken of accessory glands, such as the liver and pancreas. 



There are various ways of breaking up food physically and chemically, 

 especially the former: some of these are briefly mentioned or figured in this 

 chapter. 



Human Digestion 



In the Mouth. The mouth cavity is the vestibule of the digestive system, the 

 reception place of the food (Fig. 11.7). The teeth break it into pieces; the 

 smaller the bits, the more quickly digestive enzymes can diffuse through them. 

 In the meantime the alkaline saliva floods the mouth and pours over the food, 

 a shifting mass because it is held on the tongue, a gymnast that continually 



