Quirks of Human Anatomy

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…

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