56 LETTERS TO BROTHER JOHN. 



While the artery is in the act of coiling itself up, it 

 is becoming still more and more minute ; till, 

 having reached this second requisite degree of mi- 

 nuteness, the blood contained in it suddenly ceases 

 to be blood, and becomes saliva. This change is 

 produced by the influence of the minute nerve 

 which accompanies it. 



*At the moment when the saliva is thus produced, 

 the artery begins to lose the characteristic struc- 

 ture of arteries, and gradually acquires that of an 

 excretory duct. The saliva now travels along the 

 duct, to its termination on the inner surface of the 

 under lip, where it is discharged from the open 

 extremity of the duct into the mouth. The excre- 

 tory duct is extremely short ; for those glands are 

 mostly situated immediately beneath the skin. The 

 skin, you are probably aware, covers the inside of 

 the lips and mouth as well as the outside ; although 

 on the inside it is much finer, and more delicate. 



This is the way in which all secretory glands, 

 except one, are formed : that one is the liver. , 



The size of the secretory glands is extremely 

 different, varying from the wonderful minuteness 

 of the ceruminous glands of the ear, whose office is 

 to secrete the wax and which are, I believe, the 

 smallest glands in the body to the great magnitude 



