182 ANIMAL BIOLOGY. [Part I. 



branous partition, while with a solution of sugar this diffusion 

 (or dialysis) takes place with extreme readiness, we shall see the 

 physiological importance of this change set up by ptyalin. 



A little thin starch mucilage may be prepared by rubbing a 

 few grains of starch into a paste with cold water, and then 

 boiling with more water. In such a solution the presence of 

 the starch may be shown by the deep blue coloration with a 

 solution of iodine. If now a little saliva be added to some of 

 the mucilage in a test tube, and the mixture be kept for some 

 time at a temperature of about 38C., the starch will be con- 

 verted into sugar. Iodine will no longer give a blue coloration. 

 At an earlier stage it may give a violet coloration, due to the 

 mixture of the starch-blue with a claret colour, due to the action 

 of iodine on dextrin. The presence of the sugar may be shown 

 by adding a few drops of a solution of copper sulphate, and then 

 a little potassic hydrate, when, on boiling, an orange-red pre- 

 cipitate of cuprous oxide will be formed. If, however, the solu- 

 tion containing starch and saliva be acitlified so as to contain 

 about *1 per cent. HC1, the starch will remain unchanged. Acid 

 prevents the action of the ferment if it does not destroy it. 



Thus we see that saliva contains ptyalin and mucin, and the 

 question arises, how are these secretions formed in the salivary 

 and mucous glands'? The gland-cells are bathed on the one 

 hand in the plasma of the blood, and on the other hand their 

 secretion is poured forth into the lumen of the tube which leads 

 to the duct. Is the secreted ptyalin or mucin simply extracted 

 from the plasma, or is it manufactured out of the materials of 

 the plasma *? The latter seems the more probable view. They 

 seem to be formed by a process of chemical change or meta- 

 bolism in the gland-cell. Moreover, it is probable that they are 

 not formed directly, but that, during the resting condition, the 

 protoplasm of the cell, in and through its special vitality, elabor- 

 ates the plasma into an intermediate substance, from which the 

 mucin or ptyalin will be elaborated in the active stage. We 

 may call the special intermediate substance mother of mucin, 

 or mother of ptyalin ; but to such intermediate substances in 

 general the term mesostates has been given. 



