OF DIGESTION. 



101 



with the veins, forming also in their course several glandular 



masses, as seen in a portion of intestine connected with a vein, 



in Fig. 54 ; and it is not until thus 



taken up, and mingled with the 



circulating blood, that any of our 



food really becomes a part of the 



living body. Thus freed of the 



nutritive portion of the food, the 



residue of the product of digestion 



passes on to the large intestine, 



from whence it is expelled in the 



form of excrement. 



211. The organs above described constitute the most es- 

 sential for the process of digestion, and are found more or less 

 developed in all but some of the radiated animals; but there 

 are, in the higher animals, several additional ones for aiding 

 in the reduction of the food to chyme and chyle, which render 

 their digestive apparatus quite complicated. In the first place, 

 hard parts, of a horny or bony texture, are usually placed about 

 the mouth of those animals that feed on solid substances, which 

 serve for cutting or bruising the food into small fragments 

 before it is swallowed ; and, in many of the lower animals, 

 these organs are the only hard portions of the body. This 

 process of subdividing or chewing the food is termed masti- 

 cation. 



212. Beginning with the Radiata, we find the apparatus 

 for mastication partaking of the star-like arrangment which 

 characterizes those animals. Thus in Scutella, (Fig. 55,) 

 we have a pentagon composed of five triangular ja^s, con- 

 verging at their summits towards a central aperture v, 'nch 

 corresponds to the mouth, each one bearing a cutting plate or 

 tooth, like a knife-blade, fitted by one edge into a cleft. The 

 five jaws move towards the centre, and pierce or cut the ob- 

 jects which come between them. In some of the sea-urchins, 



9* 



