102 ANIMAL FUNCTIONS. 



instead of being perpendicular, as was supposed, is lateral, 

 and at the same time somewhat circular, so that th. 

 power it exerts, though immensely great, is directed nearly 

 in the plane of the grinding surfaces, and thus that the 

 sharp edges and points were bent or broken by a grinding 

 motion and not by direct force. 



290. But this does not account for some of the results 

 observed in the appearance of the sharp points, which 

 were worn off, as if rubbed on a stone. This effect was 

 at first attributed to the acrid c r solvent juices of the organ ; 

 but, as was afterward proved, is really the effect of the 

 pebbles which are always found in the gizzards of birds, 

 when they can be obtained. No doubt now exists among 

 naturalists, but that these pebbles are absolutely necessary 

 to the perfect digestion of the food, the action of the giz- 

 zard alone being insufficient to reduce its contents to the 

 proper state for that process. 



291. After the food has been prepared by the gizzard, 

 it passes on to the stomach, where, by a process to be here- 

 after described, digestion and assimilation are performed. 



ORGANS OF NUTRITION AND VITALITY IN THE MAMMALIA. 



292. The mammalia include the highest orders of or- 

 ganic development in the animal kingdom, embracing, as the 

 term signifies, all such animals as nurse their young, as the 

 human species, the quadrupeds, quadrumanna or monkey, 

 and whale. 



293. The animals are provided w^ith a complication of 

 organs, all of which are more or less subservient to the 

 process of digestion, for without this process none of the 

 other functions could long be sustained. The heart and 

 arteries w r ould in a short time cease to act, unless they 

 were supplied with blood, and the blood being formed 

 of chyle, would cease to be produced, if the process 

 of digestion, by which the chyle is elaborated, should 

 be suspended or destroyed. With the cessation of 

 arterial action, the functions of the brain and nerves 

 would fail, and thus life itself would become extinct. 



What a..,mals does the class mammalia include? What other functions 

 depend on that of digestion? If the action of the heart and brain should 

 cease, how would digestion be effected ? What is said of the dependance 

 of these functions on each other? 



