It is a matter of very little consequence in conversation whether a man has a large or a small mouth, as there are no means of changing that which nature has allotted us. True philosophy teaches us therefore to be thankful for such as we have got, always provided that we get something to fill it with, honestly. There is nothing more absurd than the endeavour of some people when speaking to make their mouth appear small. The result is generally the most insufferable grimace, which it is possible to conceive, disfiguring a countenance. When one is told some extraordinary news it is natural enough to open the mouth and give vent to some such exclamation as oh ! or ah ! But then you must observe the manner in which you do so, and not allow every one to see the interior of the mansion with its garniture of teeth, jawbones, and other accessories. In laughing, it is much better to do so with the lips than to open the mouth wide to give vent to those great guffaws which often accompany great mirth. When you hear a story which interests or amuses you, you must guard against opening your mouth too widely. If ever you should have occasion to read in company, whether prose or poetry, melodrama, or classic tragedy, avoid by all means any approach to those convulsive movements of the mouth that are sometimes seen in people devoid of taste. And finally, you must always keep at a respectable distance from the person with whom you are conversing, as it is possible that you may be surprised by a sudden sneeze or unsuspecting cough, or by some other accident which might cause your friend to consider the propriety of providing himself with his umbrella the next time you accosted him.
Source: How to Shine in Society, or, The Art of Conversation: Containing its usages, laws, rules, application, and examples. (Glasgow: George Watson, 1860), 13-14.