Book One -- The things that sustain and support the entire body, and what braces and attaches them all. [the bones and the ligaments that interconnect them] | Chapter 4 On the Structural Relationships of Bones

When Nature formed arthrodia 33

It is as if Nature employed this type of articulation in a simple joint where she decided the bone would move scarcely a little bit; for you will nowhere find a simple and authentic arthrodia where motion is obvious. Likewise you may observe a double arthrodia capable of obvious motion, but it will be a virtually single and simple motion. The first vertebra [atlas] of the neck will be proven to make no other movement than a clear rotation [articulatio trochoidea] over the second [axis], and at the same time the first vertebra of man is joined to the second at two points (join Y in fig. 4 ch. 15 with c in fig. 5; then a, b in fig. 4 with d, e, f, in fig. 5; finally L in fig. 4 with g in fig. 5) [articulatio atlanto-axialis mediana et lateralis] by arthrodia: on each of the sides of the dens of the second vertebra, 34 and the dens itself is in a fashion connected [ligamenta alaria] to the first vertebra by a kind of arthrodia. 35 Yet in these vertebral connections, the hollows [facies intervertebrales] and the heads are still more obvious than in the simple arthrodiae that we mentioned before. In addition, you will observe this kind of arthrodia in the remaining vertebrae of the neck and all the vertebrae of the thorax (join X and Y [fovea costalis superior] in figure 3, Chapter 16 with a and b [fovea costalis inferior] of figure 2 or c of figure 3) as far as the one that I shall explain is received by the vertebrae above and below it. 36 The connections of these vertebrae [articulationes zygapophysiales], which are located near the root of the spine or lower process of the vertebrae, are so joined together that the portions of one vertebra that enter another swell out very gently, and the parts admitting the other vertebra are hollowed only on the surface. But in these vertebrae some motion to the sides 37 is observed besides the flexion and extension. Thus if we have said that this kind of double articulation of the vertebrae with each other is arthrodia, we will admit that it is not always made for the sake of a completely simple motion.



Book One -- The things that sustain and support the entire body, and what braces and attaches them all. [the bones and the ligaments that interconnect them] | Chapter 4 On the Structural Relationships of Bones