Because, as we become weary, the nervous impulses which direct the respiratory movements are enfeebled. It has been said that those movements are involuntary, and that the parts engaged in producing them are not subject to fatigue. But the operation of breathing is, _to some extent, voluntary_, though when we cease to direct it voluntarily, it is involuntarily continued by organs which know no fatigue. When, therefore, we feel weary--still controuling our breathing in our efforts to move or to speak--there frequently arrives a period when, for a few seconds, the respiratory process is suspended. It seems to be the point at which the voluntary nerves of respiration are about to deliver their office over to the involuntary nerves; but the pause in the respiration has caused a momentary deficiency of breath, and the involuntary nerves of respiration, coming suddenly to the aid of the lungs, cause a spasmodic action of the parts involved, and _a yawn_, attended by a _deep inspiration to compensate for the cessation of breathing_, are the result. [Verse: "And it shall be, when they say unto thee, Wherefore sighed thou that thou shalt answer, For the tidings, because it cometh; and every heart shall melt, and all hands shall be feeble."--EZEKIEL XXI.] 1023. _Why do we cough?_ Because the respiratory organs are excited by the presence of some body foreign or unnatural to them. A cough is an effort on the part of the air tubes to free themselves from some source of irritation. And so important are the organs of breathing to the welfare of the body, that the muscles of the chest, back, and abdomen, _unite in the endeavour to get rid of the exciting substance_. 1024. _Why do we sneeze?_ Because particles of matter enter the nostrils and excite the nerves of feeling and of smell. In sneezing, as in coughing, the effort is to free the parts affected from the intrusion of some matters of an objectionable nature. And in this case, as in the former one, there is a very general sympathy of other organs with the part affected, and an energetic effort to get rid of the evil. 1025. _Why do we sigh?_ The action of sighing arises from very similar causes to those of yawning. But in sighing, the nervous depression is caused by _grief_; while in yawning, it is the result of _fatigue_. In sighing, the effect is generally erased by an _expiration_--in yawning by an _inspiration_. The mind, wearied and weakened by sorrow, omits for a few seconds to continue the respiratory process; and then suddenly there comes an involuntary expiration of the breath, causing a faint sound as it passes the organs of the voice. 1026. _Why do we laugh?_