One inquires further whether laughter is only proper to a human.
1. And it seems not. For laughter arises from touching the diaphragm. But the diaphragm exists in other animals, just as it does in the human. And therefore, etc.
2. Moreover, tickling the underarms and the palm of the hand and the sole of the feet causes laughter. But these parts, or ones analogous to them, exist in other animals. Therefore, etc.
The Philosopher says the opposite.
In addition to this, one can inquire why touching or tickling the underarms or the sole of a foot, rather than other parts, causes more laughter.
To the first, one must reply that laughter arises immediately from the cleanness of the blood and the spirits' thinness. Therefore, because the spleen is the receptacle for melancholy, which is the dregs of the blood, it is said to be the principle of laughter, because the spleen cleanses the blood when it attracts the melancholy. But in other animals the blood is very thick, and this is why laughter does not exist in other animals.
The reason this occurs more in the underarms than in other parts is that the underarms are network of pores [emunctoria] for the heart and partially adjoin the inner diaphragm. Therefore, an inordinate motion in them at once causes motion in the diaphragm and, likewise, warms it.
And this occurs in the human more than in other animals on account of the thinness of his skin. This arises on the sole of the foot and the palm of the hand because these parts are more nerve-filled and have a good sense of touch. This is why, etc.
Moreover, unaccustomed things are very quickly perceived, and the soles of the feet are not accustomed to being tickled or stroked, and this is why tickling the sole of the foot causes laughter more readily than tickling the hand or other parts of the body.
Moreover, a reason why a person can endure his own touch on these parts rather than that of others is this: namely, habituation and trust of nature. For when parts are based in the same root, the nature of one part trusts another just as it does itself. But when they are not based in the same root, an affliction of the part arises as if it were distrustful of the other, and this is why a person bears his own touch on many parts on which he does not bear the touch of another. And another cause is habituation, which is almost like another nature or which can be changed into nature, according to the Philosopher in the second book of the Ethics.
Thus one must respond to the question that the cleansing of the blood in a human is the reason why laughter is properly present in him.
1. To the argument one must reply that merely touching the diaphragm is a cause of laughter only when it is accompanied by clarity in the blood, because the purer the blood, the more it is diffused to the parts and causes joy.
2. To the next argument one must reply that tickling can occur in other animals owing to an unaccustomed touch, or it may happen sometimes that some of them may have thin skin, etc., yet laughter does not occur, etc.
Therefore, it is well known that when a woman is stroked on her breasts and belly, this arouses her then for intercourse, and, although she may hide the fact, she burns then like fire. And this happens for this reason, that her testicles located in the womb hang down on certain nerves and cotyledons coming from the breasts and the belly. Thus when the breasts are touched, the sperm is driven out of them and they pour it out into the womb. This effusion into the womb arouses a soporific warmth in her, just like water cascading over limestone. For this reason, women particularly desire intercourse then. Therefore, at that time, because of the vehemence of the desire, they secretly urinate and emit sperm and sometimes cleanse themselves. Therefore, crafty suitors, as I heard in confessions in Cologne, tempt women with this kind of trick and touch. Women who seem to reject such things, if truth be told, desire them all the more and intend to consent, but in order to appear chaste they deny such things, etc.
-  In DA, the term "cotyledon" is used primarily to indicate lobules on the placenta that receive nourishment from the veins. It is also commonly used to indicate the point of juncture where a stem meets a piece of fruit. Some similar point of intersection must be envisioned here.