THE ZOOLOGICAL POSITION OF THE DOG. 13 



gen derived from without, while the poisonous ingredients 

 that have found their way into the blood are got rid of 

 through processes that may be, in general, compared to 

 those of a sewage system of a very elaborate character. 

 To explain this regeneration of the blood in somewhat 

 more detail, we must first consider the fate of food from 

 the time it enters the mouth till it leaves the tract of the 

 body in which its preparation is carried on. 



The food is in the mouth submitted to the action of a 

 series of cutting and grinding organs worked by powerful 

 muscles ; mixed with a fluid which changes the starchy part 

 of it into sugar, and prepares the whole to pass farther on 

 its course. When this has been accomplished, the food is 

 grasped and squeezed and pushed along the tube, owing 

 to the action of its own muscular cells, into a sac (stom- 

 ach), in which it is rolled about and mixed with certain 

 fluids of peculiar chemical composition derived from cells 

 on its inner surface, which transform the proteid part of 

 the food into a form susceptible of ready use (absorption). 

 When this saccular organ has done its share of the work, 

 the food is moved on by the action of the muscles of its 

 walls into a very long portion of the tract in which, in 

 addition to processes carried on in the mouth and stom- 

 ach, there are others which transform the food into a con- 

 dition in which it can pass into the blood. Thus all of 

 the food that is susceptible of changes of the kind de- 

 scribed is acted upon somewhere in the long tract devoted 

 to this task. But there is usually a remnant of indigesti- 

 ble material which is finally evacuated. How is the pre- 

 pared material conveyed into the blood ? In part, direct- 

 ly through the walls of the minutest blood-vessels distrib- 



