Next it rushes a hurry call to the muscles controlling your lungs and throat, and says, "Howl!" and you howl accordingly.

Another jab at the switchboard, and the eyes are called up and ordered to weep, while at the same time the muscles of the trunk of your body are set in rhythmic movement by another message, and you rock yourself backward and forward.
This weeping and rocking yourself backward and forward and nursing your foot seem rather foolish,--indeed you have perhaps often been told that they are both foolish and babyish,--but, as you say, you "can't help it," and there is a good reason for it.

The howl is a call for help; and if the hurt were due to the bite of a wolf or a bear, or the cut had gone deep enough to open an artery, this dreadfully unmusical noise might be the means of saving your life; while the rocking backward and forward and jerking yourself about would also send a message that you needed help, supposing you were so badly hurt that you couldn't call out, to anyone who happened to be within sight of you.

So that it isn't entirely babyish and foolish to howl and squirm about when you are hurt--though it is manly to keep both within reasonable limits.
If the message about the thorn had been brought by your eyes,--in other words, if you had seen it before you stepped on it,--then a similar but much simpler and less painful reflex would have been carried out.

The image of the thorn would fall on the _retina_ of the eye and through its _optic nerve_ the message would be flashed to the brain: "There is something slim and sharp in the path,--looks like a thorn." When this message reached the brain, and not till then, would you see the thorn, just as in the case of the pain message from the foot.