310 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



and tessellated pavement (Fig. 197), such as are found in 

 skates. Pavement teeth are used for crushing shells 

 before swallowing. They are interesting as the sim- 

 plest form of teeth, and as showing their origin from 

 the gum. They are a secretion on the surface of the 

 gurri, and afterward fixed to the subjacent bone. 



SECTION III. 

 Stoniack Digestion ; Chymification , Peptonization. 



As already seen, after mastication and insalivation, 

 the food is gathered by the tongue into a bolus and 

 pressed into the throat (so much is voluntary) ; then 

 seized by the involuntary muscles and rushed down 

 through the gullet (oesophagus) into the stomach, to 

 undergo there the second stage of preparation. The 

 oesophagus is a muscular tube about ten inches long and 

 nearly an inch in diameter. The muscular fibers are 

 mostly circular or ring fibers. * We have already said 

 that a characteristic of involuntary muscles is a con- 

 secutive contraction of fibers, producing propagated 

 waves of contraction in one direction. Such a wave of 

 contraction propagated downward carries the food be- 

 fore it to the stomach. It is a strong, water-tight con- 

 traction, as shown by the fact that long-necked animals, 

 like the horse, swallow water upward from the ground 

 in drinking. The normal direction of waves is down- 

 ward or stomachward. These are called peristaltic. 

 Sometimes — abnormally in man, as in vomiting, but nor- 

 mally in ruminants, as in bringing up the cud — the waves 

 may run in the contrary direction. These are called 

 antiperistaltic. 



Saccharization of the Food.— This belongs to 

 mouth digestion, for the saliva is the digestive juice for 



