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Prologue: This post is about the adult (say, a 20 year old) human skeleton; I'm not particularly interested in infant skeletons.

The human spine has is composed of cervical, thoracic, lumbar, sacral and coccygeal vertebrae

enter image description here

Image source

My school textbook (which has a history of possessing erroneous/outdated information) states:

The coccyx consists of 4 coccygeal vertebrae that are fused to become a single bone.

However, this Wikipedia article states:

Most anatomy books incorrectly state that the coccyx is normally fused in adults. In fact it has been shown that the coccyx may consist of up to five separate bony segments, the most common configuration being two or three segments.

Is Wikipedia correct?

If so, what is the (most common) number of bones that constitute the spine? Is the number of vertebrae that constitute the spine going to be different?

This other Wiki article appears to be particularly contentious:

A fully grown adult features 30 bones in the spine, whereas a child can have 33.

  • The cervical vertebrae (7)

  • The thoracic vertebrae (12)

  • The lumbar vertebrae (5)

  • The sacral vertebrae (5 at birth, later fused into one)

  • The coccygeal vertebrae (5 at birth, some or all of the bones fuse together but there seems to be a disagreement between researchers as to what the most common number should be. Some say the most common is 1, others say 2 or 3, with 4 being the least likely. It is counted as 1 in this article.)

I understand that there is little to no variation in the number of cervical, thoracic and lumbar vertebrae among humans, so they altogether constitute $\mathrm{7+12+5 = 24}$ vertebrae/bones (in this case, I believe "vertebrae" and "bones" are synonymous).

Assuming that the sacrum really is one fused bone (my question's regarding the coccyx, so I'm not going to question the anatomy of the sacrum in this post) consisting of five vertebrae (I think there's a bone-vertebra distinction here, so they aren't the same thing)

In that case, the cervical, thoracic, lumbar and sacral contributions are $\mathrm{ 24 + 1 = 25}$ bones, or $\mathrm{ 24 + 5 = 29}$ vertebrae. (Right?)

According to the Wikipedia article I cited above (the second one), the adult human spine has 30 bones.

So the coccyx contributes $\mathrm{30 - 29 = 1}$ bone to the spine, or $\mathrm{30 - 25 = 5}$ vertebrae to the spine.

In other words, the second Wiki article implies that the coccyx is a single (fused) bone. However, the first article states that the coccyx is really composed of 3-5 (say, 4 on an average) bones.

This is horribly contradictory.

If the finer points of my question was lost, I'll restate them here:

1) Is the coccyx a single bone or multiple bones?

2) How many bones are there in the adult human spine?

3) How many vertebrae are there in the adult human spine?

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    $\begingroup$ Although from an academic standpoint these are critically important questions, the variability in human anatomy unfortunately renders the academic answers reasonably meaningless regarding the coccyx in any clinical circumstances. Although variability is much less, higher in the spine it is common to see a sacralized L5 or an S1 that failed to properly fuse, even more rare are the cervical ribs. Clinically, some of these items actually mean something, yet I have never seen a clinically significant issue related to coccygeal fusion. $\endgroup$
    – RudyB
    Commented Dec 26, 2017 at 3:21
  • $\begingroup$ @RudyB And that was the point I was making (with a different emphasis) in my comment below. Academically, the fusion (or lack thereof) of the coccygeal segments matters. Clinically (meaning practically), it really doesn't in most cases. $\endgroup$
    – Deepak
    Commented Dec 27, 2017 at 0:35

2 Answers 2

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The wikipedia article links to two papers. The first article has data for 120 pain-free and 51 affected patients with data on the number of coccygeal segments in each. We can back-calculate a reasonable approximation of segment distributions from their percentages:

  • 1 Segment: 7%
  • 2 Segments: 51%
  • 3 Segments: 38%
  • 4 segments: 4%

So 2 or 3 segments makes up almost 90% of the population. Given that only 7% of the subjects had single element coccyxes, I think it's safe to call the coccyx usually made of multiple elements.

Whether you refer to those multiple elements a single bone if largely a semantic argument. Since there is variation, it seems reasonable to call the collection 5 of variably fused terminal vertebrae the "coccyx".

As a practicing anatomist, I would say that there are 29 vertebrae, as in 29 vertebral elements (but you can't say 29 ossification centers, because each is made of multiple centers). These fuse into 24 free vertebrae, the sacrum (5 fused vertebrae) and one coccyx, which is variably fused.

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    $\begingroup$ It is odd that you say you don’t trust Wikipedia for anything, but then base your entire answer on a paper that Wikipedia cites without analysis on whether that paper is accurate and up-to-date, which was really the question. Wikipedia can usually (though definitely not always) be trusted to accurately report the sources it cites, but whether or not it has the best source available is a much more difficult question—the kind of question you would bring to experts to get their opinion on. $\endgroup$
    – KRyan
    Commented Dec 26, 2017 at 14:41
  • $\begingroup$ @KRyan -- kmm's Bona Fides and profile/activity/awards rather makes this answer an "expert" opinion. -- Wikipedia can be edited by anyone and the "peer review" quality varies widely, as anyone with expert knowledge in a field who has used Wikipedia knows. -- I've had my own edits rolled back when I not only cited sources but cited the legally documented source that defined the very answer I gave. i.e. I literally proved my answer right outside of any matter of opinion and still my answer was rolled back. That was the last time I contributed to Wikipedia. $\endgroup$
    – user23715
    Commented Dec 26, 2017 at 15:54
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    $\begingroup$ @user23715 I’m not doubting them, I’m saying that his answer does not so much as even assert that this is the right article to be looking at. “I am an expert and this paper is up-to-date and correct,” would be fine. But instead the answer is more like “the paper says this, which Wikipedia accurately reports,” which even I (who cannot name all of the major bones of the arms and legs) could have determined myself. $\endgroup$
    – KRyan
    Commented Dec 26, 2017 at 15:58
  • $\begingroup$ @KRyan -- I think he has that covered with, "As a practicing anatomist, I would say...", no? -- His disparagement of Wikipedia is founded IMO. Besides, if one has to "trust but verify" every time one uses Wikipedia why not bypass it to begin with and go straight to the sources? -- Something like Wikipedia made more sense when a search engine like Google did not exist. Now, failure to find appropriate sources is one's own fault. Using a squiffy source like Wikipedia as a first pass is rather redundant and their difficulties in getting funded lately means many others have figured this out. $\endgroup$
    – user23715
    Commented Dec 26, 2017 at 17:01
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    $\begingroup$ To be fair, I do think it's incongruous that it wouldn't matter to you as a "practising anatomist". Anatomy is all about definining body structure. That includes variations, both common and less common. As a practising doctor, I've studied Anatomy in med school, and I know how important minutiae about spatial relations and variations are in the subject - I mean, that's the whole point of the subject! I bet this sort of thing is even more important to academic practitioners of Anatomy. What you said may have made more sense if you'd said "physician" (or even "surgeon") instead of anatomist. $\endgroup$
    – Deepak
    Commented Dec 27, 2017 at 0:30
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lets answer in order.

  1. Is the coccyx a single bone or multiple bones?

answer yes, A bone is not a concrete definition and can be used to describe the number of elements or the number of contributing elements. Bone is more a definition of material than object. My favorite example is the fused mammalian jaw which can be one or two bones (two fused dentaries one mandible) or two bones (two fused dentaries). Even using the word elements can get confusing. Like many things in biology nature doesn't care about our attempts to define and categorize.

  1. How many bones are there in the adult human spine?

26 -35 depending on the individual and how you use the term.

  1. How many vertebrae are there in the adult human spine?

this one is easy, 34-35 for most people. The variation is becasue not everyone develops all the coccyx vertebrae. Fused vertebrae are still counted as several individual vertebrae. This is why we have a seperate term for the fused element, the sacrum or attach the descriptor "fused". This comes up a lot in paleontology since animals change the number of vertebrae they have and how they are distributed as they evolve. Sauropods are particularly known for swapping vertebrae across segments.

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  • $\begingroup$ I made a mistake on my math on question 3, I have fixed it, no stunning revelation I just added wrong. . $\endgroup$
    – John
    Commented Dec 26, 2017 at 1:01
  • $\begingroup$ I suggest using > instead of four spaces. It's HTML5-ly better, and looks nicer. $\endgroup$
    – wizzwizz4
    Commented Dec 26, 2017 at 12:42
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    $\begingroup$ It isn’t just nicer looking—proper quote boxes are also vastly better for accessibility. Which is to say, they aren’t actively terrible for accessibility, which codeboxes can be. Please never use code formatting for non-code: quote boxes, or simple quotation marks, work quite well for quotation. When you use code boxes, you may see a gray box with monospace font, but others—say, those listening to the page because they are blind—may get something very different, which may be hard to understand. Reading letter-by-letter is not unheard of! So please, format properly. $\endgroup$
    – KRyan
    Commented Dec 26, 2017 at 14:43

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