Strong feeling, mental or physical, being, then, the general cause of laughter, we have to note that the muscular actions constituting it are distinguished from most others by this, that they are purposeless.  In general, bodily motions that are prompted by feelings are directed to special ends; as when we try to escape a danger, or struggle to secure a gratification.  But the movements of chest and limbs which we make when laughing have no object.  And now remark that these quasi-convulsive contractions of the muscles, having no object, but being results of an uncontrolled discharge of energy, we may see whence arise their special characters—­how it happens that certain classes of muscles are affected first, and then certain other classes.  For an overflow of nerve-force, undirected by any motive, will manifestly take first the most habitual routes; and if these do not suffice, will next overflow into the less habitual ones.  Well, it is through the organs of speech that feeling passes into movement with the greatest frequency.  The jaws, tongue, and lips are used not only to express strong irritation or gratification; but that very moderate flow of mental energy which accompanies ordinary conversation, finds its chief vent through this channel.  Hence it happens that certain muscles round the mouth, small and easy to move, are the first to contract under pleasurable emotion.  The class of muscles which, next after those of articulation, are most constantly set in action (or extra action, we should say) by feelings of all kinds, are those of respiration.  Under pleasurable or painful sensations we breathe more rapidly:  possibly as a consequence of the increased demand for oxygenated blood.  The sensations that accompany exertion also bring on hard-breathing; which here more evidently responds to the physiological needs.  And emotions, too, agreeable and disagreeable, both, at first, excite respiration; though the last subsequently depress it.  That is to say, of the bodily muscles, the respiratory are more constantly implicated than any others in those various acts which our feelings impel us to; and, hence, when there occurs an undirected discharge of nervous energy into the muscular system, it happens that, if the quantity be considerable, it convulses not only certain of the articulatory and vocal muscles, but also those which expel air from the lungs.

Should the feeling to be expended be still greater in amount—­too great to find vent in these classes of muscles—­another class comes into play.  The upper limbs are set in motion.  Children frequently clap their hands in glee; by some adults the hands are rubbed together; and others, under still greater intensity of delight, slap their knees and sway their bodies backwards and forwards.  Last of all, when the other channels for the escape of the surplus nerve-force have been filled to overflowing, a yet further and less-used group of muscles is spasmodically affected:  the head is thrown back and the spine