BY SHEPHERD OGDEN ?The heart of winter is passing. Despite the cold here near the end of January, a part of us notices that the days are getting longer, just as sometime in late August we suddenly notice the length of those warm languid evenings. And that part of us always seems to know what it means.
The question, one might ask, is what part?
Some of our simpler fellow creatures, like horseshoe crabs, have clumps of light-sensitive cells in fairly obvious places (the crabs have them on their tails) but others, like swallows, have them embedded in the skull. That has often been the explanation used for humans: that we have a small organ, called the suprachiasmatic nucleus, which acts as a sort of master clock, ordering the function of our bodies according not only to time of year but also time of day. It lies inside the skull, just above the place where the optic nerves from each eye converge. It