Coccyx
From EvoWiki
- This article is about the caudal vertebrae of hominoids. For the similar structure in birds, see Pygostyle
The coccyx (also known as the tailbone, although it is not normally a single bone nor a fusion of seperate bones) is the name given the caudal vertebrae of apes. In adult apes, the number of coccygeal vertebrae is variable, but is usually between three and five caudal vertebrae[1]. In embryonic development, however, there are 10-12 caudal vertebrae (forming an embryonic tail), the first seven of which later die by apoptosis.
Function
The coccyx is an attachment point for several muscles of the pelvic floor: the sphincter ani externus and the iliococcygeus (where the coccyx is the sole attachment point), the coccygeus and the pubococcygeus (both of which attach to the sacrum in addition to the coccyx) and the gluteus maximus (which primarily attaches to the sacrum and ilium of the pelvis). The iliococcygeus and pubococcygeus are homologous to the iliocaudalis and pubocaudalis muscles of other mammals.
In some humans, another muscle group called the sacrococcygeus may also be present attached to the coccyx. This muscle group is a homologue of the tail musculature in monkeys and other mammals, where it serves to control the tail, but is poorly developed or even absent in most humans.
Vestigial nature
The coccyx is homologous to the caudal vertebrae that form the tail of most mammals and many other tetrapods. As it no longer can perform the usual function of a tail - providing balance and, in monkeys, grasping. Therefore, it is said to be a vestige - possessing no new function but remnants of an old function. In monkeys and other mammals, the muscles of defecation attach to the caudal vertebrae at the base of the tail. As apes lost these vertebrae, some were still preserved to fulfil this function. The structure of the coccyx is exactly what would be expected for a reduced and internalised tail.
Creationism, on the other hand, does not predict that the coccyx would bear any resemblance to a tail at all. If it was designed solely as a support for various muscles, one has to wonder why it would be composed of more than a single bone. Indeed, some people do have a completely fused coccyx, forming a single bone, and suffer no known complications because of it. In addition, why develop six or seven extra caudal vertebrae as an embryo when these will simply die later in development?
References
|
This page is part of the EvoWiki Encyclopedia of Anatomy and Physiology. |
