Niddah, Chapter 8, Mishnah 1. This chapter turns to the subject of the kesem, a stain of blood that a woman discovers without ever having felt or seen a flow. The question that runs through the chapter is when such a stain renders her tmei'ah and when it may be left in its taharah, and the Mishnah begins with stains found on her own body: which places on the flesh are treated as evidence of uterine blood, and which are not.
The first clause sets down the rule that governs everything to follow. A woman notices a bloodstain somewhere on her skin: "haro'ah kesem al besarah." The verdict hinges entirely on the location of the mark. Should it lie "kneged beis haturpa," directly opposite the opening through which uterine blood would pass, that is, in the region of her genitals, the ruling is "tmei'ah," impure, since it is reasonable to trace the stain to the womb. Should it instead appear "veshelo kneged beis haturpa," at a spot that blood from that opening could never have reached, the mark must be credited to something else entirely, and the ruling is "tehorah": her taharah is undisturbed.
With the principle in hand, the Mishnah proceeds to work through the body one location at a time. Take "al akeiva ve'al rosh gudala": the back of the foot, and the toes together with the foot's upper surface. A stain in either of those spots leaves her tmei'ah, for we reckon that blood issuing from the beis haturpa is capable of reaching there.
"Al shoka ve'al parsoseh," on her lower legs and her thighs, the ruling splits according to the side of the limb. "Mibifnim," if the stain is on the inner surface of the leg or thigh, "tmei'ah," because there too we assume that the blood could have come from the uterus. "Mibachutz," if it is on the outer surface, "tehorah," she is pure.
Finally, "ve'al hatzdadin, mikan umikan": the sides of the limb, whether the mark falls toward the front or toward the back. Every surface apart from the inner one belongs to this category, and in each such case the woman is tehorah and stays pure.
The thread that ties the whole list together is the principle with which the Mishnah opened. A stain by itself is not a sighting of blood; it becomes significant only when its location makes it reasonable to trace it to the beis haturpa. Where the geography of the body rules that out, the stain is attributed to some other source, and the woman's taharah stands.