Because very frequently _the mere sight_ of rich fruit, or acid substances, _will cause the saliva to flow freely_. In this case it is evident that the salivary glands _could not see or know_ that such substances were present. An impression must, therefore, be made upon the brain, _through the organ of vision_, and the desire to taste the substances being awakened, a nervous stimulus is _imparted to the glands of the mouth_, and they at once commence their action, _as if food were present_. 875. _Why does food descend into the stomach?_ Because, after the teeth, the tongue, and the muscles of the mouth generally, have rolled the food into a soft bolus, it is conveyed to the back of the mouth, where it is set upon the opening of the throat (_oesophagus_). It does not then descend through the throat by its own gravity, because the throat is generally in a compressed or collapsed state, like an empty tube; and we know that persons can eat or drink when with their heads downwards. The oesophagus is formed of a number of muscular threads, or rings, and _each little thread is like a hand ready to grasp at the morsel that is coming_. As soon as the bolus is presented at the top of the throat, these little muscular hands lay hold of it, and transmit it downward, passing it from one to another, until it is conveyed through the long passage, to the door of the stomach, which it enters. [Verse: "Remove far from me poverty and lies; give me neither poverty nor riches; feed me with food convenient for me."--PROVERBS XXX.] [Illustration: Fig. 49.--SECTION OF THE STOMACH, &c.] A. The inner coat of the _stomach_. (The stomach is here represented cut through its length, so that we can see its inside.) B. The lower extremity of the throat, or _oesophagus_, through which food enters the stomach. C. The passage out of the stomach, called the _pylorus_, where a muscular contraction prevents the escape of undigested food. D. The _duodenum_, and the ducts through which the _bile_ and _pancreatic_ juices enter and mingle with our food. 876. _Why do we not feel the food being transmitted through the throat?_ Because the nerves of the body differ in their powers: some are nerves of _feeling_, some of _motion_, and others are nerves of the _senses_. The nerves of feeling are most abundantly distributed to those parts _where feeling is most useful and necessary to us_. But the faculty of feeling our food undergoing digestion would be no service to us whatever; therefore the nerves of _motion_ are plentifully distributed to the throat and stomach, but very few of the nerves of _feeling_--just as many as will tell us when we eat anything _too hot_, or _too cold_, or that the stomach is _out of order_. 877. _Why do we feel uneasy after eating to excess?_ Because the stomach is _distended_, and presses upon the other organs by which it is surrounded.