Quirks of Human Anatomy by Lewis Held

Figure Legends 6 6.1 * 6.1R * 6.2 * 6.2R * 6.3 * 6.3R figure legends 1 * 2 * 3 * 4 * 5 * 7 * A N.B.: An 'R' suffix denotes reflections (commentaries, annotations, and further references) pertaining to the numbered legend that precedes it. [Select any image to enlarge; use back button to return] Fig. 6.1 Rogues� gallery of human anatomical flaws, ranging from the merely silly (a, f, j), to the patently stupid (b, c, d, e, i, k), to the potentially lethal (g, h, l, m). See Table 6.1 for further information. a. Muscles (dark bands) that permit ear wiggling, a dubious talent that has no apparent function aside from entertaining young children: (1) superior, (2) anterior, and (3) posterior auricularis [1004,1488]. b. Eyeball with optic nerve. The retina is considered to be inside out because its photoreceptors (p) lie behind its ganglion cells (g), an orientation we may have gotten stuck with early in evolution (cf. Fig. 6.2). Since then, the photoreceptors have come to rely on the (outer) layer of pigment cells (not shown) [2098], so it may now be impossible to reverse this order. Thickness of retinal layers is exaggerated. c. Blind spot where ganglion cell axons exit the eye. This suboptimal escape route is due to the inverted geometry shown in b and explained in Fig. 6.2. The hole in the retina is sizeable (~9 full moons in the sky), but we don�t notice it because (1) the blind spots of the two eyes are in separate parts of the visual field and (2) our brain automatically fills in gaps in our visual field by interpolation [2117].

d. The common crowding of human teeth�especially "wisdom" teeth, which erupt last�is traceable to the evolutionary shortening of our jaw. See text. e. Human embryo at 31 days postfertilization. Branchial arches (in dashed box) are a vestige from our fish ancestors. They now serve only as precursors for structures which could develop without such clefts (cf. Ch. 4). f. Nipples are useless in human males (cf. Ch. 5). g. Choking (upper left) is caused by malfunctioning of the epiglottis. The epiglottis is a cartilaginous flap (e) that reflexively covers the trachea (T) (lower box) when we swallow so that food or drink goes to the esophagus (E). The silhouette is a sagittal section with cavities as blank areas [612,889,2502,2737]. The line between E and T represents their adjacent lumenal walls [685]. One way to prevent choking (upper right) would be to have separate pipes for breathing and swallowing. In fact, that configuration already exists in babies, who can suckle and nose breathe at the same time: their larynx is so high that the epiglottis can touch the base of the nasal cavity at the midline [1547], with milk flowing around the juncture (see ref. [1469] for a clear diagram). The same is true for other mammals, who rest their epiglottis on the back edge of the soft palate [889,1546,1547]. The problem with having air bypass the mouth entirely, of course, is that it precludes talking [1931], although toothed whales still manage to "sing" to each other via an asymmetric specialization of their nasal passages [1718,2412]. h. Childbirth involves a tight fit between the baby�s head and the mother�s pelvic opening (cf. Fig. 7.1) [525].

Indeed, the passageway is so confining that the head must turn as shown (long axis transverse) to enter the birth canal, but then must rotate 90 degrees (long axis sagittal) to exit and let the shoulders enter [10,2220]. The process would be safer (and less painful) if the path mimicked a caesarean section, with the baby exiting through the navel. Drawn on the baby�s head are the fontanels (open sutures) that allow the skull to deform during birth [500,685]. i. Back pain often occurs during pregnancy because of strains in the lumbar region (cf. Fig. 5.1). More serious repercussions of the spine�s sinusoidal shape include herniated disks [10]. j. The vasa deferentia are tubes that conduct sperm from the testes to the urethra. They are much longer than they need to be in humans because of the circuitous route that they took evolutionarily [2384]. k. Routing a tube like the urethra through a solid organ like the prostate runs the risk of strangulation if the organ overgrows�a condition all too common in older men. l. In May-Thurner Syndrome, blood clots typically arise in the left�but not right�leg [196,320,1683]. This peculiar ischemic asymmetry is due to a normal quirk. The abdominal aorta and the inferior vena cava travel side by side to the groin. Just after they split into the two common iliac arteries and the two common iliac veins (one artery and one vein routed to each leg), the right fork of the arteries passes over the left fork of the veins. This overlap compresses the left vein between the spine and the overlying artery, resulting in occasional thrombosis. There is no such crossover on the right side. m. After ovulation, the egg leaves the ovary and enters the flower-shaped opening of the adjacent oviduct. Because fertilization occurs in the oviduct, the embryo can occasionally stick to the oviduct wall before it ever gets to the uterus, resulting in a life-threatening "ectopic" or "tubal" pregnancy [91].

It would have been safer if evolution had kept the length of the oviduct to a bare minimum or done away with it altogether and connected the ovary directly to the uterus. Fig. 6.1R The full-length man and woman are redrawn from a plaque aboard Pioneer 10�the first human-made object ever to leave our solar system [2253]. Launched in 1972, this human-sized spacecraft whipped past Jupiter (picking up speed), crossed Pluto�s orbit, and began coasting toward Aldebaran, which it should reach in ~2 million years [2125]. The plaque was meant as an interstellar "postcard." If any aliens stumble upon it, they will surely chuckle at our quirks (if they can laugh), although turnabout is fair play, and if we were to see their two sexes (or however many they have) au naturel, we might chortle right back at them. The standing humans were drawn by Linda Salzman in collaboration with astronomers Carl Sagan (her then-husband) and Frank Drake [573]. Choking diagrams (g) are adapted from refs. [1866,2331], and childbirth sketches (h) are redrawn from refs. [1579,1786,1866], except for the profile of the gravid woman, which is based on photos of harpist Cheryl Gallagher from her musical compact disc Pregnant Pause. g. As explained in the text, choking became a problem when our lungs arose as a branch of our eating tube. Fran�ois Jacob ascribed such flaws to the trial-and-error nature of the evolutionary process [1265]: "To make a lung with a piece of esophagus sounds very much like tinkering." There is also a deeper question here: why must people die when deprived of oxygen? Based on the prevalence of hibernation, estivation, and facultative anaerobiosis among animals [76,1675,2505], the answer is unclear [1171]. Sea turtles, for example, can hold their breath for at least three hours [1600].

Evolution, it would seem, could have given us the means to survive episodes of choking, drowning, or suffocation [1316,2229]. Why didn�t it? Presumably, the rarity of asphyxiation among primates (by drowning, etc.) reduced the marginal advantage that any salvational mutations might have had to a negligible level. The greatest irony about oxygen is that it used to be poisonous for living things before ~2 billion years ago [468,774], but eukaryotes evolved ways to detoxify and harness it to our metabolism so that it has become vital for us [12,152,2128]. This abrupt reversal of fortune was as dramatic as any Shakespearean plot. h. While we are decrying the pain of childbirth, we should also pity the poor kiwi, whose egg comprises 25% of her body weight [915], for she, too, labors mightily [2549]. Some quirk of ratite history (as yet unknown [342,343,2071]) doomed those birds to that lunacy [969]. The depicted solution for humans (i.e., birth through the navel) may seem farfetched, but the spotted hyena has rerouted its birth canal just as drastically: hyena pups must make a ~180-degree turn when they reach the mother�s pelvis and then exit through her clitoris (pseudopenis) [803]! The problem with this path is that the diameter of the clitoral meatus is too narrow (~2.2 cm) to pass the pup�s head (~6.5 cm), so the pain must be excruciating, which makes one wonder why hyenas laugh at all! Darwin made an interesting observation about our fontanels. He noted how lucky we are that these hinges were already in place (because of how skull bones grow) before they acquired the function of allowing our skull to deform during the tight squeeze of the birth process.

In other words, mammalian sutures were "co-opted" as hominin hinges:

The sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition ... but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals. [559] (p. 197)

Fig. 6.2 Hypothetical (bislagiatt) explanation for two flaws of human eyes: our backward retina and our blind spot. ("Bislagiatt" stands for "But it seemed like a good idea at the time.")This diagram incorporates the inversion scheme of Balfour (1881) [2736], the axonal inferences of Polyak (1957) [2061] and Sarnat and Netsky (1981) [2278], and the opacity conjecture of Walls (1942) [539,2736], all of which are based on the tendency of chordate development to recapitulate its evolution [956]. Cartoons are transverse sections. Fish fins (which evolved later) are added merely to orient the reader (dorsal up, ventral down). Notwithstanding the symbols in the key, photoreceptors (p) and ganglion cells (g) are also neurons (n), albeit part of the peripheral versus central nervous system (CNS). Cell size is greatly exaggerated. a. Our bilaterian (protochordate?) ancestor is thought to have had (1) a superficial nerve net [1458,1584,2579], (2) eyespots made of photoreceptors and ganglion cells [1461], and (3) criss-crossed wiring where ganglion cells projected axons to motor neurons (subset of n) on the opposite flank [2278]. The advantage of this contralateral wiring was that it allowed each eyespot to turn the body reflexively away from potential harm whenever it detected the shadow of a possible predator [2061,2278].