130 LETTERS TO BROTHER JOHN. 



I shall now endeavour to exhibit the principal 

 actions concerned in the nutrition of the body, by 

 tracing a portion of food through all the necessary 

 changes, until it has ceased to be food, and has be- 

 come an integrant part of yourself. 



Let us suppose you to be in the act of despatch- 

 ing a hearty meal, consisting of animal food and 

 various kinds of vegetables. You first introduce it 

 into your mouth with your teeth you masticate it 

 by means of your tongue, you roll it about your 

 mouth. This rolling about brings it in contact with 

 the several excretory ducts of the salivary glands; 

 which open on the internal surface of the mouth, 

 as we have before seen. These ducts, by virtue of 

 their SENSIBILITY, become aware of the presence of 

 a stimulus (the food). The stimulation which the 

 food in the mouth exerts upon the ducts is propa- 

 gated along them to the arteries, which, as we have 

 before seen, form the salivary glands, by being 

 coiled up into those little knots, so called. The 

 arteries, thus stimulated, are excited to increased 

 action; they bring a greater quantity of blood to 

 the glands : and those parts of the arteries which 

 constitute the glands, being also excited to greater 

 action, and having an increased supply of blood, 

 separate from that blood an increased quantity of 



