This time I encountered one of my skeletons on the world wide web as a banner of America's Earliest Museums (figure 1). It is the black rat (Rattus rattus) skeleton I have build. The black rat is a bit smaller than a brown rat, with a more slender skull and a longer tail. The tail is up to 130% of its body size!
In this photo there is something visible that has astonished me for some time: on the top of one of the vertebrae, visible above the shoulder blades, there seems to be an extra bone attached to the spinous process, at a 90 degree angle pointing towards the skull. I have encountered this extra bone in rats as well as in mice. I had to do some digging if this occurrence was documented in literature and it was described in 1984: "The splenius muscles of both sides form a bipennate muscle plate connecting the occipital bone with the spinous process of the second thoracic vertebra. The lateral parts of both muscles are attached directly to this prominent bony process, whereas the medial parts end in a median raphe which forms a tendinous cranial extension of the second thoracic vertebra. This tendinous extension, showing no connection to the cervical vertebrae, serves also for the attachment of acromio-trapezius muscle fibers." - Pfizter & Zenker.
As expected, the purpose of the bone is for muscle to attach to! There are other animals with a surprising "extra bone". Cormorants and darters have a unique bone on the back of the top of the skull known as the os nuchale or occipital style which was called a xiphoid process in early literature (figure 2). This bony projection provides anchorage for the muscles that increase the force with which the lower mandible is closed. This bone has highly developed muscles over it.
Figure 1: The black rat (Rattus rattus) skeleton.
Figure 2: A great cormorant (Phalacrocorax carbo) skull.
And well, wouldn't this be the perfect topic to mention baculums? Also known as the penis bone. It is a bone in the penis of many placental mammals. Altough not present in humans, it is present in the penises of other great apes. Why do humans lack this bone? Atough there are some evolutionary explanations, there is one story that I always like to share: It has been argued that the "rib" in the story of Adam and Eve is actually a mistranslation of a Biblical Hebrew euphemism for baculum, and that its removal from Adam in the Book of Genesis is a creation story to explain this absence (as well as the presence of the perineal raphe – as a resultant "scar") in humans. The story makes a lot more sense like that, don't you think?
Bibliography:
- Pfister, J. & Zenker, W. (1984). The splenius capitis muscle of the rat, architecture and histochemistry, afferent and efferent innervation as compared with that of the quadriceps muscle. Anat Embryol (Berl) 169(1): 79-89. doi: 10.1007/BF00300589.
- Yarrell, William (1828). On the xiphoid bone and its muscles in the Corvorant (Pelecanus carbo). The Zoological Journal 4: 234–237.
- Garrod, A. H. (2009). 1. Notes on the Anatomy of Plotus anhinga. Proceedings of the Zoological Society of London 44: 335–345. doi:10.1111/j.1096-3642.1876.tb02572.x.
- Burger, A. E. (2015). Functional Anatomy of the Feeding Apparatus of Four South African Cormorants. Zoologica Africana 13: 81–102. doi:10.1080/00445096.1978.11447608.
- Martin, R. D. (2007). The evolution of human reproduction: A primatological perspective. American Journal of Physical Anthropology 134: 59–84. doi:10.1002/ajpa.20734. PMID 18046752. S2CID 44416632.
- Ankel-Simons, F. (2010). Primate Anatomy: An Introduction. Elsevier. ISBN 978-0-08-046911-9.
- Gilbert, S. F. & Zevit, Z. (2001). Congenital human baculum deficiency: The generative bone of Genesis 2:21-23. American Journal of Medical Genetics. 101 (3): 284–85. doi:10.1002/ajmg.1387. PMID 11424148.
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