Why do we have TWO eyes and TWO ears, but only ONE nose?

M.H.

But you do have two noses! Take another look.

Your "one-nose" theory is a common misconception. There are two nasal cavities, mirror images of one another, which are separated in the midline by the nasal septum, a part-bone, part-cartilage "divider." You have two nostrils, two sets of turbinates (fleshy structures within the nose that warm, filter and humidify the air that you breathe), and two olfactory organs (which are responsible for your sense of smell). If you were to "lose" one side (due to an injury, for example), you could still breathe and smell using the other side.

There are many midline structures that seem to be single but are in reality paired. The lips, jaws, hard and soft palate and neck are all good examples of this. In each example, paired structures are fused together during early development in the womb. When errors occur during early development, we call the result a "birth defect." Cleft lip and cleft palate are the most familiar examples of birth defects in which fusion fails to occur appropriately. A cleft of the nose is also possible, but some clefts (such as a "cleft neck") are not compatible with life, so the fetus would be miscarried.

Next time you are in the public library, see if they have a textbook on embryology. Embryology is the study of early development. Embryology textbooks always have a series of figures or photographs showing development of a human baby, from fertilized egg all the way up to mature infant. Very early in development, the embryo is not even recognizably human. At the head-end of the embryo are a set of paired structures called "branchial arches." Branchial means "gill," and in fish, these do, indeed, develop into gills. In humans and many other animals, the branchial arches fuse in the midline to form the midline facial and neck structures. To the sides, the branchial arches form the ears.

Cleft lip and palate are not the only problems that occur as a result of a problem with branchial-arch development. Within each arch is a "valley" called a cleft -- not to be confused with "cleft" lip/palate, although the word has the same meaning in each case (a rift or divide between two structures). The branchial clefts are compressed out of existence during early development (except for the cleft that is thought to give rise to the ear canal). If any "bubbles" of cleft tissue remain, they can give rise to large cysts later on in life. These branchial-cleft cysts usually arise in the neck in early childhood or adolescence, but they can also occur in adults. As with cleft lip and/or palate, branchial cleft cysts can be treated surgically.