Capricorn takes the knees, and by extension the bones, the joints and the skin, following the general assignment of hard, dry and structural tissue to Saturn, the sign's ruler. The knees are the joint on which kneeling depends, and a good deal of medieval moral commentary made something of that, linking the sign of authority and limit to the posture of submission. The classical scheme reads Saturn for the cold and the dry, and complaints of the region were accordingly attributed to obstruction, stiffening and the effects of age. The pairing of bones with Saturn is one of the more persistent astrological correspondences and appears in modern popular astrology substantially unchanged, which makes it a useful case for observing how little the doctrine has altered.
In the classical planetary scheme, before the eighteenth-century discovery of Uranus, Saturn ruled both Capricorn and Aquarius, so the two consecutive regions at the end of the sequence, the knees and then the ankles, were both read under the same planet's cold, dry, structural character; only the later assignment of Uranus to Aquarius in modern astrology split the pairing. The knees have kept their Saturnine reading throughout, while the ankles, as the next article in this family notes, absorbed an additional and much later layer once the circulation of the blood became known to medicine.
Sources AstronomicaMarcus Manilius with A History of Horoscopic AstrologyJames Herschel Holden