By Elizabeth A. Murray, Mount St. Joseph University
Our bones can be categorized into four shapes: long bones, like the femur and humerus of the limbs; flat bones, like some in the skull; short bones, which are smaller, boxy-bones, like the carpals in the wrist; and irregular bones, like vertebrae, which don’t really fit any of the other categories well. Read on to know more.
Axial Versus Appendicular Skeleton
One general feature of bones is the contrast between the axial part of the skeleton and the appendicular skeleton.
The axial bones are those that make up the main longitudinal axis of the skeleton. These include the 22 bones of the skull; the 6 tiny ossicles in the ears; the single hyoid bone in the neck; the 26 bones of the vertebral column—the 24 individual vertebrae, plus the sacrum and coccyx, or tailbone as it’s called; the 24 ribs; and the sternum. That’s a total of 80 bones.
The appendicular skeleton, on the other hand, includes the 30 bones in each upper limb, the 30 bones in each lower limb, as well as the two clavicles and two scapulae of each pectoral girdle, and the pair of pelvic bones that attach our lower limbs to the axial skeleton, so the appendicular total is 126 bones. That’s 206, when the appendicular and axial totals are combined.
Bony Landmarks
There are some standard terms used for bone features—such as foramen, tubercle, condyle—and then adjectives are used to specify those on a given bone. An overarching scheme of bony landmarks would boil down to three main categories: projections, depressions, and openings.
Depressions and projections are typical of muscle attachment sites and joint surfaces, while openings will transmit arteries, nerves, and veins. Learning the general bony landmark terms in advance will help prevent confusion, since numerous bones have tubercles, notches, heads, facets, and so forth.
It must be kept in mind that some of the specific landmarks also use anatomical directional terms like lateral, medial, superior, and inferior.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Bony Landmarks: Openings
Here’s the infraorbital foramen—it’s a hole inferior to the eye socket, which is anatomically known as the orbit. Conveniently, the infraorbital nerve, artery and vein all pass through it. And by the rule of adjectives—if we have an infraorbital foramen, we also have a supraorbital foramen. Sometimes that is an incomplete opening, so many of us have a supraorbital notch, rather than a supraorbital foramen.
There is also obturator foramen, the largest foramen in the body, but despite its large size, very little passes through it.
And then there is the external auditory meatus—that’s a tube-like passageway, lined with skin, through which sound waves pass to reach the eardrum. Inside the skull, there’s an internal auditory meatus, which satisfies the rule of adjectives.
Bony Landmarks: Depressions
In terms of depressions, we have the intertubercular groove or sulcus of the humerus—you’ll hear both used. It’s called that because it’s between the greater tubercle of the humerus and the lesser tubercle of the humerus—so, since “inter” means between, and it’s a groove between those tubercles, the intertubercular groove is a good name for it!
Another depression is the glenoid fossa; that is the name for the shallow socket of the shoulder joint which the humerus fits into.
And another shallow depression is the iliac fossa, named for the part of the pelvis it’s on—the ilium. The muscle that originates here is called iliacus. As the skeleton is the framework of the body, the names of the bones are the basis for the names of many other structures. That’s why bones are typically covered first in musculoskeletal study—or really when you study any form of anatomy.
Bony Landmarks: Projections
As for projections, we mentioned the greater tubercle and the lesser tubercle of the humerus—there’s that rule of adjectives.
The scapula has a glenoid fossa, and it also has a supraglenoid tubercle above that socket—or better yet, superior to that socket—so what else do you suppose it has inferior to that socket? If you said infraglenoid tubercle, you’re right!
These large projections on either side of the elbow are the medial and lateral epicondyles—sticking out on either side of the humerus above the joint region of the elbow. So you see medial and lateral being used.
These examples should give you an idea that learning the types of bony landmarks—condyles, foramen, and facets—will make a good foundation for learning the names of specific bone features later.
Formation of Bony Landmarks
So, where do these bony landmarks come from? A usual answer is that the landmarks are there so muscles can attach to them. But that’s not exactly the case.
To some extent, these projections—trochanters, tubercles, processes—they are there because muscles attach to them. That might sound like double-talk, or a fine point, but really, it’s not. The landmarks don’t develop to facilitate the muscle attachment; they grow because bone’s reaction to stress is to build more bone matrix.
Wolff’s Law
It must be noted that the matrix is composed of the collagen fibers surrounded by calcium phosphate salts. That addition of more bone matrix in response to stress strengthens the bone. Essentially, this relates to a concept in anatomy known as Wolff’s law, which states that bone will adapt to the loads under which it’s placed.
This is why stress like weight-bearing exercise is healthy for bones—within reason, of course. It is also why inactivity causes bone atrophy—whether it’s astronauts on the space station, a bedridden patient, or a person in a cast for six weeks.
Common Questions about the Features of Our Bones
An overarching scheme of bony landmarks would boil down to three main categories: projections, depressions, and openings.
The axial bones are those that make up the main longitudinal axis of the skeleton. These include the 22 bones of the skull; the 6 tiny ossicles in the ears; the single hyoid bone in the neck; the 26 bones of the vertebral column.
The appendicular skeleton includes the 30 bones in each upper limb, the 30 bones in each lower limb, as well as the two clavicles and two scapulae of each pectoral girdle, and the pair of pelvic bones that attach our lower limbs to the axial skeleton, so the appendicular total is 126 bones.
