DIGESTION OF POOD. 



281 



the vermiform appendix of man. Adaptation has heen illus- 

 trated in the lifetime of a single individual in a remarkable 



Fia. 228.— Diagram illnstrating arrangement of intestine, nervous system, etc., in com- 

 mon snail, Helix (after Huxiey). m, mouth; (, tootti; od, odontopliore; a, gullet: 

 c, crop; «, stomach; r, rectum; o, anus; r, s, renal sac; A, heart; X lung (modified 

 pallia! chamber); n, its external aperture; em, tUck edge of mantle united with 

 sides of body; /, foot; cpg, cerebral, pedal, and parieto-splanchnic ganglia aggre- 

 gated round gullet. 



manner ; thus, a seagull, by being fed on grain, has had its 

 stomach, naturally thin and soft-walled, converted into a mus- / 

 cular gizzard. 



Since digestion is a process in which the mechanical and 

 chemical are both involved, and the food of animals differs so 

 widely, great variety in the alimentary tract, both anatomical 

 and physiological, must be expected. Vegetable food must 

 usually be eaten in much larger bulk to furnish the needed 

 elements ; hence the great length of intestine habitually found 

 in herbivorous animals, associated often with a capacious 

 and chambered stomach, furnishing a larger laboratory in 

 which Nature may carry on her processes. To illustrate, the 

 stomach of the ruminants consists of four parts {rumen, reticu- 

 lum, omasum, or psalterium, dbomasum). The food when 

 cropped is immediately swallowed ; so that the paunch (rumen) 

 is a mere storehouse in which it is softened, though but little 

 changed otherwise ; and it would seem that real gastric digestion 

 is almost confined to the last division, which may be compared 



