172 GENERAL PHYSIOLOGY 



when a foreign body comes into the throat. Further, the mucus 

 of the saliva serves to make masticated food smooth, so that the 

 masses of food can glide more easily through the narrow gullet. 

 In this lies the chief importance of the saliva in man; here, on 

 account of its too brief action, the ptyalin, which works only in 

 an alkaline liquid, and hence in the acid gastric juice is made 

 immediately ineffective, can hardly exercise its amylolytic power. 

 Finally, mucus serves for attachment, especially in the lower 

 animals and unicellular organisms. Rhizopoda secrete upon the 

 surface of their protoplasmic bodies a delicate mucous covering 

 with which they stick themselves to the bottom in order to creep 

 about, and with which also they hold fast food-organisms that 

 swim against them, in order to draw the latter into their own 

 bodies and digest them. A similar importance as protective 

 media is possessed by the fats which, such as the sebum, are 

 produced by the sebaceous glands of the skin ; they protect the 

 skin from too great evaporation and render it supple. 



Further, as Stahl ('88) has shown by a series of experiments, 

 many secretions act in a different manner solely as protective 

 media in animals and especially plants : such are ill-smelling or 

 ill-tasting adds and ethereal oils. The organisms are protected by 

 them from being devoured. Most of these cases present interest- 

 ing phenomena of adaptation to definite conditions, which have 

 arisen through natural selection and constitute contrivances 

 advantageous to the organism. The same is true also of other 

 cases in which plants, by means of good-smelling and good- tasting 

 secretions, such as ethereal oils and honey, attract insects whose 

 coming and going are useful, perhaps indispensable, to the plants ; 

 the animals bear aw r ay pollen upon their legs and deposit it 

 upon the female flowers so that the latter are fertilised. Such 

 adaptations, often astonishingly fitting, are especially common 

 among plants, and the physiology of secretion touches here most 

 closely the interesting field of the mutual relations of plants and 

 animals. 



Finally, as secretions in the widest sense there may be recognised 

 also substances produced in the cell, such as starch, aleur one- grains, 

 fat-droplets, etc., which are stored in the cell for a time as reserve- 

 material and later are used in metabolism. 



Among the secretions that after their production r&main in the 

 organism, there belong almost exclusively pigments and substances 

 that form skeletons. The former appear mostly in the form of 

 fine granules, remain continually in the cell-body, and possess a 

 special importance in the colour-changes of the animal, which is 

 not yet entirely explained. The great majority of skeleton- 

 forming substances are excreted to the outside. Sometimes they 

 are laid down within the cell itself and later extruded, as are the 

 calcareous needles and plates of the Holothuria ; sometimes they 



