36 HUMAN PHYSIOLOGY 



regarded as a little chemist, seeing that secretion 

 implies not merely a separating from the blood of 

 certain products already formed therein, but also 

 an actual process of manufacture out of the raw 

 material represented by the blood of the new sub- 

 stance or fluid. 



ABOUT SALIVA. This fluid in the case of the 

 salivary glands is a colourless fluid consisting very 

 largely of water and containing minerals, while its- 

 chief constituent is a substance or ferment known 

 as ptyalin. Upon the presence of ptyalin depends 

 the particular action which the saliva discharges in 

 the work of digestion. To saliva is assigned the 

 first step in digestion, namely, that of converting the 

 starch we eat into a form of sugar known as grape 

 sugar. This is accomplished by the action of the 

 ptyalin ferment, and we are therefore taught the 

 lesson that the starch consumed so largely as food, 

 is intended by nature to pass to the stomach, not in 

 the form of starch, but in that of a sugar. It may 

 be remembered that all throughout digestion this 

 rule holds good. Starch, as such, is practically use- 

 less to the body. It may be, as we shall see, stored 

 up in the liver in the form of starch, but when 

 required for bodily purposes to be applied to the 

 tissues as a food, and especially to the muscles as 

 an energy-producing substance, it must be presented 

 in the form of sugar. Another point worthy of im- 

 portance, because possessing a distinct bearing on 

 the nourishment of young children, is the fact that 

 ptyalin does not appear in the saliva of the child 

 until it attains the age of seven or eight months. 

 Prior to this age it is therefore incapable of con- 

 verting starch into sugar, nature making up for the 

 want of starch by placing sugar ready made in the 



