188 



DIGESTIVE APPAKATUS GASTRIC DIGESTION. 



tary tube ; and it is not easy to say where it commences and 

 terminates. In Spiders and Scorpions, too, which live upon 

 the juices they suck from other animals, the alimentary tube 

 is very simple ; and it is scarcely dilated into a proper sto- 

 mach. And in most of the Eadiated classes, we find the 

 stomach to possess only one orifice, through which the undi- 

 gested residue of the food is cast out, as well as fresh sup- 

 plies taken in. But this stomach is not always a simple 

 bag ; thus in the Star-fish it sends prolongations into the 

 rays, the use of which is at present un- 

 determined. There are certain animals in 

 which no digestive cavity exists : their 

 sustenance being derived either from the 

 juices prepared by other animals, in 

 whose tissues or cavities they are im- 

 bedded, and being introduced by absorp- 

 tion through the whole surface, as is the 

 case in the lower Entozoa (fig. 53) ; or 

 from particles which are drawn into the 

 midst of the soft gelatinous substance of 

 their bodies, and undergo a sort of diges 

 tion there, as is the case with the Rhizo- 

 poda ( 129). 



Gastric Digestion : Chymification. 



204. The food which has been re- 

 duced in the mouth by the action of the 

 teeth, or in the stomach itself by the 

 movement of its own tendinous walls, is 

 prepared for the real process of digestion; 

 by which it is converted into a fluid, 

 and thus made fit to be truly received 

 into the system, by being absorbed into 

 its vessels. The chief agent in the 

 digestive process is a fluid termed the 

 gastric juice, which is secreted or sepa- 

 as seen in a vertical section rated from the blood by a vast number 

 * C dSe^ m aJTd ^ little bags or follicles (fig. 113), im- 

 twenty diameters at B. bedded in the walls of the stomach. 

 When the cavity is empty, this fluid is secreted in very small 

 quantities ; but, like the salivary secretion, it is poured out 



Fig. 113. 

 GASTRIC FOLLICLES, 



