LETTERS TO BROTHER JOHN. 55 



raation of some particular gland and its excretory 

 duct. 



Within the mouth and its neighbourhood are 

 numerous glands, called salivary. Their office is, to 

 secrete the saliva. Three of the salivary glands 

 are of very considerable size : the rest are very 

 minute. Let us trace an artery until it forms any 

 one of 1 these single minute glands : say, one of those 

 called labial, which are situated on the inside of 

 the lips. 



One of the external carotid arteries say, the 

 right gives off several branches smaller than itself. 

 One of these branches is called the facial artery. 

 The facial gives off several branches smaller than 

 itself^ and one of these is called the submental : the 

 submental gives off several branches smaller than 

 itself. One of these very small branches goes to 

 the under lip, becoming, in its course, more and 

 more minute. Dipping into the substance of the 

 lip, and still diminishing in size as it traverses and 

 assists in forming the ultimate tissue of the lip, and 

 having at length acquired the requisite degree of 

 hairlike minuteness, it suddenly turns upon itself, 

 and rolls itself up into a little corpuscle, of the size, 

 probably, of a mustard-seed. This little body is 

 the labial salivary gland we have been in search of. 



