The HAPS Discussion group (also known as HAPS-L and before that as “the listserv”) is the place where the most interesting conversations in A&P are happening. This discussion group has hundreds of members, is very active, and has often features amazingly high level conversations among leaders in the field. This group was started in 1998 as an email listserv, and some still call it that, but it is a modern discussion group with email preferences and a web archive. The HAPS discussion group is open to all current HAPS members and is one of the most valuable perks of membership.
This week, one discussion revolved around the most accurate classification of bone types. In this discussion, Mark Nielsen (University of Utah Anatomy Professor and winner of the 2017 HAPS-Theime Excellence in Teaching Award) shared multiple illuminating contributions to the conversation. Check out the excerpt below…and then imagine having content like THIS delivered to your email box on a regular basis.
WOW, there is a lot of interesting discussion going on here, this is one of the nice things about the HAPS listserve. It is always great to share and discuss. While I agree with many of the sage comments about classification and “does it really matter because the bones do not care or know where they fit in the scheme of things”, it is still important to recognize that there is correct and incorrect within a classification scheme. Following is the bases of the classification scheme:
Long bone = what is the one characteristic shared by long bones that none of the other bone types have, one thing and one thing only, a medullary cavity, and yes all the phalanges, even the small distal phalanges have a medullary cavity, as does the clavicle. The following bones have a medullary cavity:
- clavicle
- humerus
- radius
- ulna
- metacarpals
- proximal phalanges of hand
- middle phalanges of hand
- distal phalanges of hand
- femur
- tibia
- fibula
- metatarsals
- proximal phalanges of foot
- middle phalanges of foot
- distal phalanges of foot
I believe someone stated that long bones are characterized because they have a diaphysis with proximal and distal epiphyses. This is not true. Many long bones only have epiphyses at one end and not the other. This is the case for many of the phalanges. Again, the characteristic that defines a long bone is the presence of a medullary cavity. Besides, many bones have epiphyses – for example, short bones and irregular bones have epiphyses.
Short bones = are characterized by a core of spongy bone with an outer covering of compact bone. They typically have a length, width, and depth that are approximately of equal dimensions. The carpal and tarsal bones are placed in this category.
Flat bones = the true flat bones of the body all reside in the skull, but the ribs are also often considered to fall in this category because their bone structure is similar to the flat bones of the skull. These are bones that are characterized by external and internal tables (laminae) of compact bone sandwiching dense trabecular diploe, the diploic spaces of the trabecular bone being filled with hemopoetic red marrow in the living subject. This would include the parietal bones, frontal bone, squamous portion of the occipital bone and temporal bone, sutural or wormian bones that are ossification centers that never fused with the fore mentioned bones.
Most of the remaining bones did not fit into one of these three categories. Like the short bones and flat bones all the remaining bones had an outer covering of compact bone and an internal core of spongy bone and no medullary cavity, but they were not short and they were not flat. This led to the next category that became the catch all:
Irregular bones = a variety of bone shapes consisting of an outer covering of compact bone and a central core of spongy bone, with some bone surfaces that are so flat and thin that they lack spongey bone completely e.g., the scapula, ethmoid. Most of the other bones fall in this category – vertebrae, the bones of the facial skeleton and inferior cranial vault bones, hyoid, malleus, incus, stapes, and the scapula and os coxae.
The final category is the sesamoid bones = these are bones that form within tendons. In human anatomy they are similar in bony structure to short bones but have a unique classification as sesamoid bones because of their location within tendons. In some other vertebrates they are very long slender bones within tendons.
One other recognized category is a pneumatized bone. These are bones that contain air spaces within their cores and can overlap with other categories. For example, the frontal bone is both a flat bone and a pneumatized bone. The ethmoid bone, sphenoid bone, and petromastoid part of the temporal bone are both irregular bones and pneumatized bones.
So there is a logic to classification and it is not a random thing that we can bend to our whims. We now have the choice to ignore it or teach it correctly.
So if you’re a HAPS member, by all means, join this discussion group. And if you’re not a member, JOIN HAPS so you can join the discussion group. (Then adjust your email settings, because most HAPSters have experienced the infamous “blown up email box” that results from some of the more rigorous conversations! Thankfully, executive director Peter English wrote a blog post with instructions for doing just that.)