Timaeus

Wherefore the thighs and the shins and the region of the loins and the bones of the upper and lower arm, and all our other parts which are jointless, and all those bones which are void of intelligence within, owing to the small quantity of soul in the marrow—all these are abundantly supplied with flesh; but those parts which are intelligent are supplied less abundantly—except possibly where He so fashioned the flesh that it can of itself convey sensations, as is the case with the tongue; but most of these parts He made in the way described above. For the substance which is generated by necessity and grows up with us in no wise admits of quick perception coexisting with dense bone and abundant flesh. For if these characteristics were willing to consort together, then the structure of the head would have acquired them most of all, and mankind, crowned with a head that was fleshy and sinewy and strong, would have enjoyed a life that was twice (nay, many times) as long as our present life, and healthier, to boot, and more free from pain. But as it is, when the Constructors of our being were cogitating whether they should make a kind that was more long-lived and worse or more short-lived and better, they agreed that the shorter and superior life should by all means be chosen by all rather than the longer and inferior. Wherefore they covered the head closely with thin bone, but not with flesh and sinews, since it was also without flexions. For all these reasons, then, the head that was joined to the body in every man was more perceptive and more intelligent but less strong. It was on these grounds and in this way that God set the sinews at the bottom of the head round about the neck and glued them there symmetrically; and with these He fastened the extremities of the jaws below the substance of the face; and the rest of the sinews He distributed amongst all the limbs, attaching joint to joint. And those who fashioned the features of our mouth fashioned it with teeth and tongue and lips, even as it is fashioned now, for ends both necessary and most good, contriving it as an entrance with a view to necessary ends, and as an outlet with a view to the ends most good. For all that enters in and supplies food to the body is necessary; while the stream of speech which flows out and ministers to intelligence is of all streams the fairest and most good.