Objection 1. It would seem that the woman should not have been formed from the rib of man. For the rib was much smaller than the woman's body. Now from a smaller thing a larger thing can be made only--either by addition (and then the woman ought to have been described as made out of that which was added, rather than out of the rib itself)--or by rarefaction, because, as Augustine says (Gen. ad lit. x): "A body cannot increase in bulk except by rarefaction." But the woman's body is not more rarefied than man's--at least, not in the proportion of a rib to Eve's body. Therefore Eve was not formed from a rib of Adam.
Objection 2. Further, in those things which were first created there was nothing superfluous. Therefore a rib of Adam belonged to the integrity of his body. So, if a rib was removed, his body remained imperfect; which is unreasonable to suppose.
Objection 3. Further, a rib cannot be removed from man without pain. But there was no pain before sin. Therefore it was not right for a rib to be taken from the man, that Eve might be made from it.
On the contrary, It is written (Genesis 2:22): "God built the rib, which He took from Adam, into a woman."
I answer that, It was right for the woman to be made from a rib of man.
First, to signify the social union of man and woman, for the woman should neither "use authority over man," and so she was not made from his head; nor was it right for her to be subject to man's contempt as his slave, and so she was not made from his feet.
Secondly, for the sacramental signification; for from the side of Christ sleeping on the Cross the Sacraments flowed--namely, blood and water--on which the Church was established.
Reply to Objection 1. Some say that the woman's body was formed by a material increase, without anything being added; in the same way as our Lord multiplied the five loaves. But this is quite impossible. For such an increase of matter would either be by a change of the very substance of the matter itself, or by a change of its dimensions. Not by change of the substance of the matter, both because matter, considered in itself, is quite unchangeable, since it has a potential existence, and has nothing but the nature of a subject, and because quantity and size are extraneous to the essence of matter itself. Wherefore multiplication of matter is quite unintelligible, as long as the matter itself remains the same without anything added to it; unless it receives greater dimensions. This implies rarefaction, which is for the same matter to receive greater dimensions, as the Philosopher says (Phys. iv). To say, therefore, that the same matter is enlarged, without being rarefied, is to combine contradictories --viz. the definition with the absence of the thing defined.
Wherefore, as no rarefaction is apparent in such multiplication of matter, we must admit an addition of matter: either by creation, or which is more probable, by conversion. Hence Augustine says (Tract. xxiv in Joan.) that "Christ filled five thousand men with five loaves, in the same way as from a few seeds He produces the harvest of corn"--that is, by transformation of the nourishment. Nevertheless, we say that the crowds were fed with five loaves, or that woman was made from the rib, because an addition was made to the already existing matter of the loaves and of the rib.
Reply to Objection 2. The rib belonged to the integral perfection of Adam, not as an individual, but as the principle of the human race; just as the semen belongs to the perfection of the begetter, and is released by a natural and pleasurable operation. Much more, therefore, was it possible that by the Divine power the body of the woman should be produced from the man's rib.
From this it is clear how to answer the third objection.
Source: Summa Theologica (New Advent)