106 ANIMAL FUNCTIONS. 



is lateral, and at the same time somewhat circular, so 

 that the power it exerts, though immensely great, is di- 

 rected 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. 



But this does not account for some of the results ob- 

 served 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, or solvent juices of the 

 organ ; but as was afterwards proved, is really the ef- 

 fect of the pebbles which are always found in the giz- 

 zards 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 gizzard alone being insufficient to re- 

 duce its contents to the proper state for that proce ss. 



After the food has been prepared by the gizzard it 

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

 after described, digestion and assimilation is performed. 



ORGANS OF NUTRITION AND VITALITY IN THE MAMMALIA. 



The Mammalia include the highest orders of organic 

 developement, in the animal kingdom, embracing as the 

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

 the human species, the quadrupeds, quadrurnanna, or 

 monkey, and whale. 



These animals are provided with 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 arid arteries would in a short time cease to act, 

 unless they were supplied with blood, and the blood be- 

 ing 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 animals does the class mammalia include 1 What other func- 

 tions 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 

 dependence of these functions on each other ^ 



