Friday, October 24, 2014


The structure of the human nose is nicely intricate, bony parts in pairs with fancy names like “the perpendicular plate of the ethmoid bone and the premaxilla bone and the palatine bone, the floor of the nose and the roof of the mouth” . . .  Then the nasal septum is composed of the “quadrangular cartilage; the vomer bone; the perpendicular plate of the ethmoid bone, which is part of the orbit of the eye; aspects of the premaxilla, and the palatine bones. Each lateral nasal wall contains three pairs of nasal conchae, which are small, thin, shell-form bones called 'turbinates'" after the turbulent air of intake and exhale.  If one is punched straight on the end of the nose, there is a danger of those shattered bones piercing the brain and thus killing the person.

“The ethmoid has three parts: cribriform plate, ethmoidal labyrinth, and perpendicular plate. Between the orbital plate and the nasal conchae are the ethmoidal sinuses or ethmoidal air cells, which are a variable number of small cavities in the lateral mass of the ethmoid.  The ethmoid articulates with fifteen bones:  four of the neurocranium and eleven of the viscerocranium—, two nasal bones, two maxillae, two lacrimals, two palatines, two inferior nasal conchae and the vomer.  The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and form, by their junction, 'the bridge' of the nose." An intact skull has many small fragile bones in the “nose hole,” and these are the ethmoid.  The hole itself is the pyriform (pear-shaped) aperture.

"The Nose", an opera derived from Gogol.

If these intricate little bony passages become infected, the result is what we call a “sinus infection” and can be of disabling intensity. The paired structures grow towards the middle from the sides.  If they fail to join, the result may be various degrees of cleft palate.  Fetal alcohol syndrome may cause them to grow together in a mismatch grotesque enough for people to say “monster.”  Much is made of the size and shapes of noses, so much so that contemporary persons of means will have theirs surgically shaped, even at the risk of losing one of the most valuable of the senses, that of smell.

The fifteen-bone ethmoid contains magnetite which birds and other migratory creatures use as compasses responding to the earth’s magnetic field.  This function was previously thought to be vestigial in humans, but maybe it isn’t.  The ethmoid is directly against the brain and if it breaks or separates enough to let cerebral spinal fluid leak through, an infection can get into the brain.  If the olfactory nerve is severed, the sense of smell will be lost.  Wikipedia understates:  “this is not fatal.”  

But even surgeries meant to relieve the pain of sinus pressure can be delicate and problematic.  “To plan, map, and execute the surgical correction of a nasal defect or deformity, the structure of the external nose is divided into nine aesthetic nasal subunits, and six aesthetic nasal segments, which provide the plastic surgeon with the measures for determining the size, extent, and topographic locale of the nasal defect or deformity.”

Barbra Streisand

“The form of the nasal subunits — the dorsum, the sidewalls, the lobule, the soft triangles, the alae, and the columella — are configured differently, according to the race and the ethnic group of the patient, thus the nasal physiognomies denominated as: African, platyrrhine (flat, wide nose); Asiatic, subplatyrrhine (low, wide nose); Caucasian, leptorrhine (narrow nose); and Hispanic, paraleptorrhine (narrow-sided nose). The respective external valve of each nose is variably dependent upon the size, shape, and strength of the lower lateral cartilage.”  People obsess about their noses because of stigmas attached to these markers.  Only recently, partly because of charismatic and elegant public figures like Barbra Streisand, have “hawk noses” become acceptable, though no one calls them “paraleptorrhine” and wearing a mask with a beak-nose will get a person into trouble with certain populations.  Even Barbra buckled and got a nose job.

"At four weeks of gestational development, the precursors of the nose begin their migration from the posterior towards the midface. Two symmetrical future olfactory epithelium develop inferiorly, which the nasal pits then divide into the medial and the lateral future upper lip and nose). The medial processes then form the septum, the philtrum, and the premaxilla of the nose; the lateral processes form the sides of the nose; and the mouth forms from the the anterior ectodermal portion of the alimentary tract (the mouth) which is inferior to the nasal complex."  We recognize each other by our faces -- our noses and what surrounds them.

“At ten weeks of gestation, the cells differentiate into muscle, cartilage, and bone. If this important, early facial embryogenesis fails, it might result in anomalies such as choanal atresia (absent or closed passage), medial nasal clefts (fissures), or lateral nasal clefts, nasal aplasia (faulty or incomplete development), and polyrrhinia (double nose).”

“This normal, human embryologic development is exceptionally important — because the newborn infant breathes through his or her nose during the first 6 weeks of life — thus, when a child is afflicted with bilateral choanal atresia, the blockage of the posterior nasal passage, either by abnormal bony tissue or by abnormal soft tissue, emergency remedial action is required to ensure that the child can breathe.”  I assume that if the baby arrives with two noses, that's not good either.   This story has surprised many people with hopeful news, because it appears that nerve cells from the olfactory bulb can actually cause a severed spinal cord to regrow, join, and function.

Olfactory bulbs in gold.

"In most vertebrates, the olfactory bulb is the most forward part of the brain. In humans, however, the olfactory bulb is on the bottom side of the brain. The olfactory bulb is supported and protected by the cribriform plate of the ethmoid bone.  The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb.  The main olfactory bulb has a multi-layered cellular architecture. In order from surface to the center the layers are:

1.  Glomerular layer In mammals, glomeruli typically range between 50-120 µm in diameter and number between 1100 and 2400 depending on the species, with roughly between 1100 and 1200 in humans.  The number of glomeruli in a human decreases with age; in humans that are over 80 they are nearly absent.  This is why my aged mother over-salted the food and used so much perfume one did not necessarily enjoy being in a closed automobile with her unless very fond of that scent.

2.   External plexiform layer

3.  Mitral cell layer is connected to the amygdala, a structure with major sensory and emotional function.  “Mitral cell output is not a passive reflection of their input from the olfactory nerve.”  In humans it is related to “tufted cells”, but separate.  In lower animals the mitral and tufted cells are merged, so the separation appears to be an evolution for some reason.  Mitral and tufted cells project to various targets in the brain. “Most importantly, projections target the olfactory cortex, where odor information can be integrated with input from other sensory modalities and used to drive behavior.”  This is part of what we call the connectome or "functional system".

4.  Internal plexiform layer

5.   Granule cell layer

The glomeruli layer of the olfactory bulb is the first level of synaptic processing.  The glomeruli layer represents a spatial odor map organized by chemical structure of odorants like molecular structure and carbon chain length. This spatial map is divided into zones and clusters, which represent similar glomeruli and therefore similar odors. One cluster in particular is associated with rank, spoiled smells which are represented by certain chemical characteristics.” 

The descriptions go on -- a great deal is known, but it is astoundingly new to understand that neurons constantly grow and that injecting neurons from the olfactory bulb can actually make a severed spinal cord regrow and integrate.  I assume that nerves from the olfactory bulb were used because if the bulb were damaged in the process, it would not be fatal.  We are not told if the ability to smell was damaged.  Would you rather walk -- or smell your lover?

No comments: