(1) The physical phenomena of laughter are not easily described, chiefly for the reasons that there is so much tumult in them, and that they are so widespread. It is not necessary, however, for my purpose to attempt any minute description. It will be sufficient if I show their character broadly.

They may be said to consist of a succession of convulsive movements effected by the combined action of the muscles of inspiration, expiration, and voice, causing a remarkable commotion, and giving the whole body special aspects and attitudes. The vocal cords are brought together and separated in rapid dancing fashion, as Dr. Wyllie explained to me, with explosive, reiterated, and differently modulated sounds as the result. In the opinion of many the diaphragm is the muscle primarily affected. In a broad fashion this is a description of the phenomena of voiced or audible laughter.

Sir Charles Bell gives his picture of the phenomena thus: 'Observe the condition of a man convulsed with laughter. He draws a full breath, and throws it out in short, interrupted, and audible cachinnations. The muscles of the throat, neck, and chest are agitated; the diaphragm is especially convulsed. He holds his sides, and from the violent agitation he is incapable of a voluntary act.' Every feature of his face is wrinkled and contorted.

Speaking roughly, these are the physical phenomena of sonorous or audible laughter. They are seated mainly in the trunk of the body, but they are always accompanied by a commotion of the muscles of the face, especially of those of the lips and mouth. The features are broadened or stretched out from the middle line, and there is a transverse stretching of the mouth, which is nearly always kept more or less open.

In this sonorous laughter the commotion of the muscles of the trunk is invariably attended by a commotion of the muscles of the face, but the commotion of the facial muscles may occur alone, without accompanying respiratory and vocal phenomena, and it then constitutes silent laughter or smiling, which is largely an affair of the physiognomy. But this silent laughter must not be regarded as loud laughter aborted. It is neither the beginning nor the end of voiced laughter. It is one part, of laughter, and it may occur without the other part. It differs from voiced laughter, not in kind, but only in degree. The smile is always present in voiced laughter, but there may be smiling without sonorous laughing. Laughter, as distinguished from smiling, is sonorous, and in its production the muscles of inspiration, expiration, and voice are in tumult. Smiling, on the other hand, is silent, resides in the face, and depends on the agitation of its very numerous muscles.

Wyllie says that the emotions find their natural expression in the language of vocal tone, facial aspect, and bodily gesture. Whatever emotion voiced laughter expresses appears to need all three, and, if silent laughter expresses the same emotion, then the muscles of the face alone are sufficient for its expression, without the aid of vocal tone or bodily gesture. We thus have two ways, fairly well differentiated, of expressing the same emotion or the same mental state.

(2) The physical phenomena of .sonorous laughter are accepted as the bodily expression of a particular mental state, and it is a very remarkable thing that any particular mental state should be expressed by such a widespread muscular commotion and tumult. It becomes still more remarkable in view of the fact that laughter, having the same physical phenomena, may occur, which we cannot regard as having its origin in any special state of mind, that is, as starting from any mental feeling or emotion. 'Laughter of this kind is excited by what is clearly physical in its character, and it is almost always voiced, though sometimes it may not go beyond silent smiling. It is produced by what we call tickling, that is, by bringing the fingers, or some other thing that is suitable, into light contact with certain parts of the skin, passing quickly from one spot of it to another. The laughter which follows tickling is not the expression of a mental state coincident with, or antecedent to, the act of tickling. It is induced by a physical operation.

(3) We thus appear to have two laughters, which are widely differentiated from each other by the nature of that which seems to cause them, but which do not differ at all in the physical phenomena which attend them. Both exhibit the same muscular perturbation of the trunk and face, the same sonorousness, and the same gesticulations and attitudinising, and nothing of all this, in either kind of laughter, comes into existence by an order of the will. It is altogether involuntary. It has not been acquired. It presents itself as instinctive. We do not learn to laugh. We do it by inheritance.

We go through the same odd and ridiculous actions, out of all proportion to their cause, whether that cause be a special state of mind or the mechanical tickling of the armpit. The performance seems as purposeless as it is strange - its extent, violence, and grotesqueness making it very remarkable.

(4) Referring to that kind of laughter which appears to be the expression of a mental state, and accepting it as largely, if not entirely, beyond control, let us suppose that a fit of such laughter, instead of being of quite short duration, as is usually the case, should last for a day, or days, or longer, then the state of mind which it expresses would be of a corresponding duration, unless we suppose that the expression outlives the thing expressed. In such a ease I venture to say that it would be difficult to say that the person thus persistently and uncontrollably laughing was not in a state of mental disorder - keeping always in view the singular and irrational character of the phenomena of laughter. When this prolonged laughter had ceased, I think we should be justified in calling what had happened a transitory fit of mental disorder.