154 ANATOMY AND PHYSIOLOGY. 



fact being that a man can swallow nearly as well when standing on 

 his head as on his feet. This will be understood if we suppose an 

 imaginary division of the gullet into a number of rings. When one 

 ring contracts, the food passes down into the next, then the second 

 contracts, and squeezes it down into the third, while the first being 

 still contracted, prevents it from getting up, and so the process goes 

 on, regularly downward, until the ball arrives at the stomach. And 

 in vomiting, an action takes place precisely the reverse of this, and 

 the food is squeezed up from the stomach into the mouth, although 

 so rapidly as to seem almost instantaneous. 



TONGUE. 



The tongue is fixed to the back of the chin, and has a muscle 

 arising from this point and radiating through it, forward, upward, 

 and backward, which can protrude the tongue, turn it upward, 

 downward, or to the side, render its surface convex, or hollow to 

 serve for a conduit, as in drinking. There are three pairs of mus- 

 cles actually forming the substance of the tongue, and not less than 

 six pairs more which can aid in its motions. The whole inner sur- 

 face of the mouth is lined with a soft mucous membrane, so called 

 because it pours out a mucus from its surface to lubricate it, to 

 protect it ? and to assist the food, to slide easily through it. The 

 upper surface of the tongue is covered with many delicate papillae 

 or points in which the nerves of taste end, which vary in appear- 

 ance in different animals. In the cow, for example, they are much 

 rougher than in man ; in the lion they are so rough as to be capable 

 of peeling one's skin off should he attempt to lick it ; and in some 

 of the marine animals which swallow living shell-fish, both the 

 tongue and the gullet are covered with thickly set spines, directed 

 backward, to prevent their prey from actually creeping up again. 



SALIVARY GLANDS. 



Six glands are placed about the mouth for the purpose of supply- 

 ing saliva to be mixed with the food; two very large ones lie 

 behind the ear, in the hollow between the lower jaw and the tem- 

 poral bone, so that the motion of eating squeezes out their contents.. 

 Their ducts run forward in the cheek, and perforate the mouth 

 opposite the second last tooth in the upper jaw, where with the 

 tongue a small soft projection may be recognised. Two others lie 

 on each side under the tongue, having a common duct, which may 

 be seen opening on the fold of membrane that bridles down the 



