DIFFUSION 



1796 



DIGESTION 



and on the plumage of some birds. It is called 

 diffraction of light, a term which means break- 

 ing away jrom. Diffraction is caused by inter- 

 ference with the waves of light. See LIGHT. 



A plate of glass prepared for the purpose and 

 ruled with at least 2,000 lines to the inch ex- 

 hibits a beautiful play of colors. Such a plate 

 is known as a diffraction screen. It may be 

 made by ruling on glass covered with gold leaf 

 or a coating of India ink and glue. A study 

 of diffraction led to the invention of the 

 spectroscope (which see), and it has also been 

 of great assistance in the study of the spec- 

 trum. See SPECTRUM ANALYSIS. 



DIFFUSION, diju'zhun, is the mixing of 

 two fluids when brought in contact. Fill a 

 glass fruit jar two-thirds full of water, then 

 by means of a pipe inject to the bottom of the 

 jar a little red ink. Within a short time all 

 the water will have a red tint; this thorough 

 mixing is diffusion. A dark-colored syrup can 

 be used in place of the ink, but it will act 

 more slowly. Gases diffuse more rapidly than 

 liquids. Were it not for diffusion the different 

 gases in the atmosphere would arrange them- 

 selves in layers; the carbon dioxide, or car- 

 bonic acid gas, which is the heaviest, would be 

 at the bottom, the oxygen would be next, and 

 the nitrogen would be at the top. See AIR. 



Diffusion of gases is easily illustrated in a 

 chemical laboratory by the following experi- 

 ment: Fill one flask with chlorine gas and 

 another with hydrogen, taking care to close 

 the mouth of each jar so no gas can escape. 

 Then connect the jars by means of a glass tube 

 so that the jar containing hydrogen will be 

 inverted over that containing chlorine. Al- 

 though the hydrogen is much the lighter, 

 some of it will descend to the lower jar, and 

 some of the chlorine will rise to the upper 

 jar. The change would take place even 

 through a thin plug of plaster of Paris. 



DIGESTION, dijes'chun. The food which 

 is taken into the mouth, however well cooked 

 and palatable it may be, is not in such condi- 

 tion that it may be absorbed at once by the 

 blood and used to nourish the body. Various 

 changes must take place, some in the mouth, 

 some in the stomach, and some in the intes- 

 tines. The process by which these changes 

 are brought about is known as digestion. 



The Mouth. Digestion begins in the mouth, 

 where the food is ground into fine particles by 

 mastication and mingled with saliva, which 

 moistens it and changes some of the starch 

 to maltose. Thus thorough mastication serves 



a two-fold purpose; it mixes saliva with the 

 food and grinds the latter so the fluids of the 

 stomach and intestines can more readily act 

 upon it. When mastication is not thorough 

 there results indigestion, or failure of digestion. 



The Stomach. 

 The masticated 

 food passes down 

 the aesophagus in- 

 to the stomach, 

 where further 

 changes take 

 place. The gas- 

 tric juice is min- 

 gled with the 

 food by a peculiar 

 churning motion, 

 caused by the 

 contraction of the 

 muscles of the 

 stomach. The 

 gastric juice 

 starts the diges- 

 tion of the pro- 

 teins, or nitrogen- 

 bearing foods, CHANNEL OF DIGESTION 



such as eggs, lean BeI( "Y ^ e mouth, the proc- 

 00 ' esses of digestion are related 



meat and milk, to the following organs : 



A ferment called <> Stomach 



(6) Duodenum 

 renmn curdles the (c) Small Intestine 



< d > Termination of the ileum 

 (e) vermiform appendix 



(now obsolete) 

 Ascending colon 



, 

 (/) 



m ;il, *A n *A a 

 milk and aids m 



its digestion. The 



starches, sugars (g) Transverse colon 



and fats are not (7l) r 



changed by the gastric juice. The time re- 



quired for digestion in the stomach varies from 



two to four hours, according to the nature of 



the food. The partially digested food, known 



as chyme, passes on into the intestines. 



The Intestines. The last stage of digestion 

 takes place in the small intestine. Here the 

 bile and pancreatic juice are mingled with the 

 chyme. These fluids act upon the fats, starches 

 and proteins; the digestion of proteins is com- 

 pleted, and also that of starch, which is begun 

 in the mouth. The fats are changed into a 

 soapy substance. The intestinal juices com- 

 plete the work of forming chyle, the fully di- 

 gested food which is ready to be absorbed by 

 the lymphatics and carried to the blood vessels. 

 There is little or no digestion in the large 

 intestine. W.A.E. 



Kelated Subjects. The following articles In 

 these volumes bear a more or less close relation 

 to the topic of digestion: 

 Absorption Alimentary Canal 



