The Skeletal System model of learning

In an earlier response I offered examples within the muscle system. Here I do the same, but with the skeletal system. Again, as before, students actually learn and enjoy learning, as well as the challenges presented by my teaching approach.
Excerpted from “A&P:Understandable & Enjoyable, Finally!” By: Peter Karch, PhD
“…..To properly study the skeleton, you are going to need the following equipment:
- A disarticulated human skeleton
- A fully articulated human skeleton
- A pictorial guide to help you associate a picture of a bone with its name (Ask your instructor to suggest some to choose from.)
- A fully articulated snake, cat, and bird skeleton. (Make no mistake about it, these are of utmost importance in understanding the human skeleton.)
Hopefully you can use a real human skeleton, or at least have access to a real, fully articulated human skeleton, so you can feel, hence marvel at, the beauty of design, especially the strength to weight ratio we discussed earlier. It is truly amazing to behold. The human skeleton constitutes only 18% of the human’s body weight. (That of a bird, the epitome of skeletal design, only 10%)! Pick one up and compare it to an artificial skeleton, probably also present in your lab. Do you agree how amazing it is?
Now: What’s with the snake, cat and bird, you ask? Any question you could possibly have about skeletal design, seriously, ANY QUESTION WHATSOEVER, can be answered by simply sitting in front of these four skeletons (the snake, cat, human, and bird) and “asking” them questions, and “listening” to their answers. Yes. Asking and listening to skeletons of snakes, cats, humans, and birds. (There are many ways to “ask” and “listen”.)
The modifications to the snake’s skeleton due to the presence of limbs, IS the cat, i.e. a quadruped. Conversely, remove the limbs and their attaching structures from a cat, and you have a snake. This one concept poignantly emphasizes everything we have been talking about in this chapter, indeed, this book: form follows function. Now take that basic quadrapedal modification, and walk bipedally, using your forelimbs as a bird or human. So which: the human or the bird? What a difference in forelimb design! BINGO! You have the best teacher you could possibly ask for! LISTEN TO THEM ANSWER YOUR QUESTIONS!
Again, I feel the laboratory is the most essential part of any science education because labs afford the individual the pleasure of, the challenge of, and the rewards of, self — discovery. There is no substitute for self — discovery in my opinion.
Asking someone for the answer to your question is easier, of course, but where are you then? You have cheated yourself of the knowledge of learning about your own potential. It is not only learning about your potential, but actually helping this ability you really do possess to mature into its full potential. That’s wrong. That is the responsibility of education, and the educator. Regard this as the same type of challenge, with the same pride of principle and accomplishment that you would, were you working with a jigsaw puzzle, or crossword puzzle, or “the puzzle” you will inevitably face on your job, in your chosen profession, alone some day. That day will come! Bet on it! Who will help you then? Will you trust that person’s answers over your own well-thought-out conclusions? Why? Why not your own? Were your answers, in fact, well thought out? Work on them!
I am going to present the bones of the skeletal system to you in two ways (“A” and “B”), each one helping you understand the other approach, both contributing to you truly understanding the skeletal system, as a system, as opposed to simply memorizing this/that bone for the impending exam, then promptly forgetting what you “learned” because the probability of this material ever showing up again in your life is zilch.
Frankly I think “Approach B is the best approach to learning the bones of the skeleton, but for you, at this stage of your understanding, I will approach it in a more traditional manner first. Thus I will teach you the names of the bones and their features (“Approach A”) before talking about the logic of these features (“Approach B”). In that way, I can use the vocabulary developed in “Approach A” to aid the flow of conversation, confident that you already know the features I am directing your attention towards, during “Approach B”.

For brevity purposes here I am just going to copy sample pages for the study of the skeletal system so you get a feel for what students are doing in the lab.
Approach A
The axial skeleton includes the head and trunk regions of the body. The following lists the names of the bones, the number of them in the normal human body (in parentheses), and a brief description of the more important features found on each bone.
I. SKULL (80 bones) — Bones comprising the “HEAD”. Except for the MANDIBLE, all bones are joined together with SUTURES (see end of “skull” section for list of sutures). You should trace the three-dimensional outline of each bone mentioned by following the sutures with a dissecting probe. (Never use a pencil or pen to touch a bone for fear of marking the bones.) Take care when handling real bones, as the smaller processes and thinner bones break easily. BEAUCHENE SKULLS (“Exploded Skulls”) are quite useful, especially in the initial stages of study, however, you should be able to identify all skull bones and markings on fully articulated skulls as well, by simply following the sutures. A. CRANIUM (8) — Those bones comprising that portion of the DORSAL CAVITY encasing the BRAIN.

1. FRONTAL (1) — — the anterior, superior portion of the skull — forms a portion of the SUPERIOR ORBITAL PLATE
- supraorbital margins contain SUPRAORBITAL FORAMINA (sometimes appear as NOTCHES) — parts form anterior portion of cranial floor inside cranial cavity.

(Do you see how important directional terminology is? Were you following the instructions by “listening” to the words? Were you identifying the particular structures by their names? For example: Where do you think you would find/look for the “supraorbital foramen”?)…………”
“……..Even though vertebrae differ from one another in several respects, two things are evident in most animals:
- They are sufficiently similar (and sufficiently different) from all other bones to discuss common features of vertebrae in general.
- They are sufficiently different from one another to warrant organizing (studying) them as members of 5 distinct groups: CERVICAL, THORACIC, LUMBAR, SACRAL, COCCYGEAL.
A. TYPICAL FEATURES OF VERTEBRAE- examine any vertebra excluding the atlas, sacrum and coccygeal bones. (These are atypical vertebrae; look at them later, after you have studied “typical” vertebrae. Only then will you be able to see similarities which have been modified by different functional needs.)
-BODY (CENTRUM) — The major, rounded, weight — bearing portion of the vertebra, situated anteriorly in the erect human, therefore ventrally in quadrupeds.
-VERTEBRAL (SPINAL) FORAMEN — The inferior (posterior) portion of the dorsal body cavity in which the SPINAL CORD is located.
-PEDICLE — The bony extensions forming the lateral wall of the vertebral foramen immediately posterior to the body.
-LAMINA — The bony extensions forming the posterior wall of the vertebral foramen between the TRANSVERSE PROCESS on each side and the singular, centrally located SPINOUS PROCESS.
-INTERVERTEBRAL FORAMEN — The specific shape of each pedicle in adjacent vertebrae creates this foramen. The 31 pairs of SPINAL NERVES exit the vertebral foramen at these sites.
-TRANSVERSE PROCESS — The lateral bony extensions at the junction of the pedicle and lamina on each side.
-SPINOUS PROCESS — A singular, bony projection extending dorsally and medially from each vertebra. It is what is felt when one rubs their fingers down one’s “spine”.
-SUPERIOR AND INFERIOR ARTICULATING SURFACES — paired FACETS located on the superior or inferior surface of each vertebra. The SUPERIOR ARTICULAR FACET of any given vertebra articulates with the INFERIOR ARTICULAR FACET of the next more anterior vertebra. The shapes of these facets, together with the shapes of the centrum of each vertebra, contribute to the shape of the vertebral column as a whole (curvatures), and allow for the flexibility exhibited by the vertebral column, hence the trunk. Students should place any two adjacent vertebrae together so they “fit properly” to see the above; note which surfaces actually articulate.



- First, most superior (anterior) seven bones of the vertebral column. — Possess all the typical features mentioned above (excluding the atlas). — Most delicate of all the typical vertebrae.
- All (and only these vertebrae) have a TRANSVERSE FORAMEN located in each transverse process.
- The first two cervical vertebrae have unique features for their unique roles among the vertebrae in general:

- no body
- superior articular facet elongated and irregularly shaped relative to inferior articular facet. (With what does the superior articular facet articulate? Why is this bone called the “atlas”? Why is the superior facet shaped differently that the inferior facet? What type of motion occurs at the superior facet?)
- Inferior articular facet more rounded or oval than the superior facet. Why? With what does this surface articulate? What type of motion occurs here?

- Second cervical vertebra
- Identify the ODONTOID PROCESS (DENS). Note its shape and location of its smooth surface. Why is it called the “axis”? What is the function of the dens? Note the shape of the superior articular facet. Why that shape? What type of motion occurs around the dens? How does the shape of the superior facet facilitate the motion occurring around the dens?

2. THORACIC (12)
- Immediately inferior to cervical vertebrae. — All typical features of vertebrae.
- All thoracic vertebrae (and only these vertebrae) have COSTAL FACETS on their bodies (a DEMIFACET on each of two adjacent vertebra: POSTERIOR DEMIFACET on more anterior vertebra + ANTERIOR DEMIFACET on more posterior vertebra = HOLOFACET into which the head of each rib articulates.)………………..”

Bones of the APPENDICULAR SKELETON make up the APPENDAGES (LIMBS) as well as the structures needed to attach the appendages to the trunk (GIRDLES). Note that the word “appendage” means “something extra”, “something added”, etc. Limbs are paired structures, two anteriorly, two posteriorily….added to the major part of the body, the trunk,…via the girdles.

Consider this:

Of the 206+ bones in the human skeletal system, approximately 126 (over half) occur in the appendages. Why should this be true? What has this to do with the word “dexterity”? How does dexterity help an organism survive? How do numbers of parts affect one’s repertoire of behaviors? The human hand has been described as “the most versatile instrument on earth”. Of the 64 bones comprising the forelimbs, 48 of them are located in the hands alone! Does this put things in perspective for you? Are you fascinated yet?

I. FORELIMB, (64) (“Anterior Appendage, Upper Extremity, Arm, Upper Limb”) — Review the general structure of the limbs discussed earlier in this chapter. The bones comprising the forelimb are as follows: SCAPULA, CLAVICLE, HUMERUS, ULNA, RADIUS, CARPUS, METACARPUS, PHALANGES.


- CLAVICLE — An “S” shaped bone which articulates with the ACROMION of the SCAPULA posteriorly and MANUBRIUM of the STERNUM anteriorly. (NB.: sternal and acromial ends.)

In the human and other primates, the clavicle serves as a bone — to — bone attachment for the pectoral girdle (hence the forelimb) to the AXIAL SKELETON. This is of utmost importance to organisms which support their weight from their arms while swinging through trees. Yes, humans are indeed “swingers”!

It is interesting that the clavicle of the cat, a specialist at running and reaching, is almost vestigial. Since it is a small, amorphous, free floating bone, the pectoral girdle (and therefore the forelimb) of the cat is not attached to the axial skeleton by way of bone at all. Only muscles hold the limb in place. This provides the cat with the ability to extend the motion of the forelimb anteriorly, which is obviously so important for the long strides of running, or the extended reach needed to grab prey with its claws.

Similarly, the horse, another specialist at running, and therefore requiring a long stride, has no clavicle at all. The presence of a clavicle actually hinders the anterior range of motion of the forelimb.

If you ever have the opportunity to watch a side view of a cat or horse running at full stride, you will immediately see the free — moving, rotary motion that the scapula undergoes during the power and recovery phases of the stride. It is beautiful and fascinating to watch. It clearly shows the need for the uninhibited motion that can only be achieved without the bone to bone attachment of the clavicle. (Living things are truly the epitome of design and function, after all, they have had 3.5 billion years of “trial and error” — “selection” — resulting in the design we see today! Take some time to observe it. . . to marvel at it. . to respect it . . . to understand it…….”

At the start of this chapter I drew a picture of a “typical long bone”, but by drawing it as I did, i.e., with a “bump” here, and a “depression” there, etc., quite inadvertently, I made it closely resemble a femur. The point I was making was that those bumps and depressions didn’t come there accidentally. Indeed, those particular bumps and depressions, at those particular places, on this particular bone, from this particular person, didn’t come there accidentally. They are the result of this bone’s function in that particular place, in that particular person, over that particular time span, i.e. all of the stresses placed on that bone over time.
In order to understand this idea better, let’s now consider a “potential” long bone of a nondescript shape, found in the proximal forelimb of a human, and let’s “watch” it perform, so we can understand its actual final shape. While there is much more actually involved than what I am discussing here (genetics and evolutionary heritage, for example), my point will be made with the following.
Figure 4–11 shows this nondescript “potential” long bone.

I am depicting it already elongated with squared-off ends only because I must draw it some way, this being as good a shape as any other, initially, especially since I designated it as being a “long” bone. Just keep in mind now, until we “get rolling”, that the bone is non descript… amorphous.
Let’s assume “A” is proximal, “B” is distal. Limbs being an appendage (APPENDICULAR SKELETON) to the trunk (AXIAL SKELETON) must have a point of attachment with the axial skeleton. This end is termed PROXIMAL; the other end, distant from this point of attachment, is termed DISTAL.
(Are you listening to these words? As I‘ve said to you many times already, vocabulary is of utmost importance in learning this, or any other new language, obviously, and biology is as certainly a new language as is Latin, Spanish, French, etc.
“Axial” = axis = major part onto which the “appendage” is attached. “Approximate”, “in proximity to”, refers to the nearness of the point of attachment of the appendage to the axis; “distal” = “distant” = the other end must be “quite distant” from this point of attachment.)
In studying the range of animals that exist on Earth, I have noticed (for reasons that are beyond the scope of this book and course), that:
1. Human limbs are mostly lateral structures; the trunk is therefore medial to the limbs. (This is true in other mammals as well, even though they appear more ventral in these organisms.)
2. Animal limbs attach to the axial skeleton by a specialized structure termed a GIRDLE.
3. The parts of the animal limbs distal to the girdles are constructed very similarly: a singular bone most proximally (articulating with the girdle), which articulates distally with two bones, themselves articulating even more distally with multiple bones (a structure termed the MANUS = HAND OR PES = FOOT).
4. Animal limbs, viewed as a whole, seem to taper distally, i.e. they are more massive proximally, and thinner, more delicate, (and numerous) distally. If the limb as a whole gives this impression, then the individual bones comprising the limb must contribute to this total. N.B.: Although the sum of the limb bones appears to taper moving distally, the surface area these tapering bones cover has increased significantly. (Remember that surface area increases as a structure is subdivided.) N.B.: Not all individual bones manifest this phenomenon of tapering, but many do. Thus, if a particular bone does demonstrate tapering, then we should be able to recognize proximal and distal ends by this factor. If the particular bone doesn’t demonstrate tapering, then this phenomenon is not useful in identifying proximal and distal ends; other methods must be employed to discover which end is which. Figure 4–11 should now look something like this (Figure 4–12)
From a posterior/anterior perspective, the human humerus does not conform to the tapering concept just described. However, from lateral view (either side), it does indeed tend to taper. Hence I would say that the broad, rounded end is proximal, the other end, is distal. If you are not convinced of the tapering exhibited by this bone, ask yourself what other aspects of this bone might allow you to determine proximal and distal ends. What about the adaptations for articulation, as you know the articulations of this bone to be?
Remember (from that list of observations made on animal limbs): we now know that the proximal end of the first limb bone articulates with the girdle, the distal end with two other bones. (First, most proximal, limb bones are singular.) From the same source, we also know that the proximal articulating surface must be located medially (a lateral structure must articulate with a more medial structure on its medial surface.)
Figure 4–13 shows what has just been discussed. The arrows point to the necessary articulating surfaces.

What type of specialized structure should there be for articulation at “A”? What about at “B”? Doesn’t the structure of the girdle suggest that the articular surface of our bone should be rounded (ball — like) so as to fit into the socket on the lateral surface of the girdle? This would provide the freest motion possible for this limb at this junction. (In support of this logic: as I watch humans using their limbs, I realize that the joint in question allows them extremely free motion relative to the other joints in the body. Thus, I would expect the anatomy of this joint to be built accordingly.) A BALL AND SOCKET joint certainly not only meets the needs at this site in the body, but can also provide the range of motion I see humans experiencing at this joint. (N.B.: I did not say it had to be a ball and socket joint, but rather that such a joint could meet the needs described.)
Besides the HEAD of the humerus, which allows free motion in the GLENOID CAVITY of the SCAPULA, I see numerous projections on the humerus and scapula. For example: the GREATER AND LESSER TUBERCLES, CORACOID and ACROMION PROCESSES. Why are they there? Earlier we learned that in order to have a movable joint, the two bones must be somewhat separated from each other to allow movement, yet they must also be held together relative to one another. The muscles necessary for movement must attach to the two bones such that they pass their energy of contraction across the moveable joint. In doing so, for any given action one bone moves relative to the other. (Actually both bones move relative to each other, however, usually one bone moves more than the other for any given action. We will discuss this concept in greater detail in the muscle chapter.)
Do you remember the strength of contraction statistics mentioned in the Histology chapter? For a muscle to attach to the bones it is moving, and not rip loose when exerting forces such as these, there must be a firm attachment to the bones. Tubercles, tuberosities, etc., i.e. the rough projections from the bone’s surface, are sites of muscle/ligament attachment. They are raised and rough because that increases contact area, which increases the binding potential in the same way that rough surfaces improves gluing potential (Figure 4–14 “A” vs. “B”). Raised areas also change the angle that a muscle acts through while causing an action, which may increase the lever action of that movement (Figure 4–15 “A” vs. “B”).

N.B.: The 2 October, 2009 issue of Science (vol,326; #5949, pgs 1–188) was entirely devoted to the newly discovered human fossil, Ardipithecus ramidus. The remains of 35 separate individuals, dated at 4.4 mya, “Ardi” being one female, were discovered in the mid 1990’s, and finally, formally presented to the scientific community by the series of 11 research articles in Science on this October date. The fossils were discovered in the Afar Valley area of Ethiopia, and pre-date “Lucy” by nearly 1 million years. Although I am introducing her here, in the Skeletal chapter, because of a bone important to our current discussion, I will bring her up two more times in both the Digestive and Reproductive chapters because of other features she demonstrates relative to those topics.
Her genus, “Ardipithecus”, refers to a “ground ape”. The species name, “ramidus”, refers to the significance of her “branch”, because of her mosaic of traits which fits her evolutionary position as an ancestor of the human lineage of primates.
What is important to our present discussion in this chapter (see above regarding angles of muscle attachments to bone protuberances), is that she walked bipedally on flat ground, but her feet, pelvis, legs and hands suggest she was quadrapedal when moving in the trees. Her big toe is significantly displaced laterally, hence prehensile, as is that of chimpanzees, but differing from chimps, she has a small bone imbedded in the tendon of the big toe that would keep this toe more rigid relative to the rest of the foot, thus serving for propulsion in bipedalism.
Notice that this is exactly opposite in effect to the surface structure necessary at the articulation itself, where the goal is to actually reduce contact area, hence the smooth surfaces. (REMINDER: smooth surfaces are places where movement occurs, whereas rough surfaces are places where firm attachment, “no movement” occur.)

What types of specialized structures are required at the distal end of the humerus? It will be remembered that the general animal limb has the singular, proximal bone articulating with two other bones distally. Two types of actions occur at the elbow in humans. (Again, to discuss why two types, and only these two types, gets too far afield for this course, although anatomists, anthropologists, archaeologists do have ideas and data on the subject.) However, the fact that two types of motion do indeed occur here is easily substantiated: watch yourself move. The types of motion occurring at the elbow are FLEXION and EXTENSION of the antebrachium on the brachium, as well as PIVOTING of the hand (called PRONATION and SUPINATION). (N.B.: At this point in the book, you may not know the full meanings of these words. Please look them up now, so we can continue our discussion, trusting me that we will spend considerable time in the Muscle chapter describing them.) You might be asking, indeed, should be asking, why two bones are needed here instead of one? Indeed, why multiple bones distally? We already answered this question earlier, didn’t we? In general, multiplicity provides a tremendous increase in surface area. Remember, the function of the limbs is to support the organism’s weight and the amplification of the weight’s force occurring as a result of motions (e.g. running, jumping, etc.) The same information about force per unit area reduction applies to the entire skeleton as applies to the shapes of the individual bones of the entire skeleton. Remember my example of 70 Kg force being applied on a one, vs. two square cm. surface area? In the former situation the force is 70 Kg/; in the latter situation it is only 35 Kg/ In addition to reducing the applied force, multiple bones also provide a considerable degree of dexterity to the limb, hence enabling the organism the ability to manipulate its environment more effectively. (N.B.: While present day humans no longer use their forelimbs to support their body weight, evolutionary evidence strongly suggests that their present forelimb structure is derived from limbs that did function in a support capacity, and therefore, would have been constructed accordingly. Besides, supporting one’s body weight is only one way of manipulating one’s environment. Certainly you can see how effectively humans have manipulated their environment using their forelimbs in quite different behaviors than simply supporting their body weight.) Returning to the original question: what type of specialization might be found at the distal end of the humerus to accommodate the two bones it articulates with, moving through the range of motions normally seen in humans at this joint? Observe the behaviors on your own body: both distal bones (the RADIUS and ULNA) move together during flexion and extension, but only one, the radius, rotates over the other more fixed bone, the ulna, during pronation/supination. Upon returning from the flexed position to anatomical position, extension, but not hyperextension can occur (although there are considerable differences in degrees of hyperextension among people). (N.B.: please trust me with these words referring to motions for now.) Thus the shape of the distal end of the humerus must be able to allow: A. Both bones to flex and extend, B. Prevent the hyperextension of the antebrachium, C. Provide a means of rotation for the radius over the ulna,
D. Allow both flexion and rotation, or extension and rotation to occur as individual functions, as well as simultaneous functions, yet flexion and extension themselves cannot occur simultaneously, since they are opposing actions.

I’m going to stop here. I’m not going to answer this question for you, but rather ask you to answer it yourself. Find the three bones presently under discussion: the humerus, radius, and ulna. Use the disarticulated skeleton to help you. (DO NOT USE THE ARTICULATED SKELETON AT THIS TIME.) You will be able to identify the bones at this site using the pictures provided in your lab book. See if you can put them together properly, such that the resulting structural complex called the “elbow joint” can accommodate all the behavioral data mentioned above. (Remember about right and left limbs.)
After piecing the joint together, check yourself with the articulated skeletal models around the lab. DO NOT ASK YOUR INSTRUCTOR TO HELP YOU until you have exhausted all possible thoughts and combinations” ….until you are at “your wit’s end”!
Then move on to another bone, or set of bones at some other particular articulation. For example: THE FEMUR, THE ETHMOID COMPLEX, THE VERTEBRAE, ETC.
Again: There is a functional reason why bones assume their shapes. This aspect of osteology is equally important to learn as the more traditional aspects of the subject. Indeed, knowing this aspect of the subject will help you with the other, more traditional, aspect of the subject.

Why stop now, since we are “on a roll”, as they say?
1. What modifications of the skeletal system would you expect were the organism to take up flight (without an airplane, of course)? Check your thoughts with the bird skeleton found in the lab. (Do this AFTER you have given the subject serious thought, not before. Who are you fooling?)
2. What factors might contribute to the vertebral column of the normal human becoming increasingly more massive as one proceeds caudally to the sacrum? Is this related to some of the thoughts you considered regarding flight?
3. Why is the pelvic girdle more massive than the pectoral girdle? Is this true in the cat as well as the human? Is that what you would have expected just looking at the cat? If there are differences between pectoral and pelvic girdles in both cats and humans, are the differences proportionately the same? Why? Why not?
What factors might contribute to the pelvic girdle of the human male being shaped as it is? What about the human female’s pelvic girdle? Are they shaped the same? Are there any differences? Enumerate these differences? Why do they exist?

Had I asked the question as follows, first, how would you have answered this question: “What factors contribute to the shape of the human female’s pelvic girdle?” Would you have answered THIS question the same way you answered the one about the human male? Careful! This is tricky! Think!

While these questions might at first appear to be in the realm of comparative anatomy, and therefore, not directly pertinent to the interests of the allied health career student, further thought will reveal that using the comparative anatomy APPROACH to answering one’s questions can lead to some profound observations about relationships between physiology and anatomy of the human skeleton. As long as comparing the anatomy and physiology of various animals doesn’t become an end in-and-of-itself, it is a very valuable tool, which the allied health student cannot afford to shy away from.
Indeed, if you really want to be fascinated, even thrilled, with learning and understanding this skeletal system, look at a mole’s scapula, specifically the spine and acromion. Then look at a bird’s sternum. Comparing these to another animal, e.g. a human, will reveal how important bones are to muscle attachments so muscles can maximize their movement potential.
I don’t intend to get too involved with physics in this observation simply because we all intuitively understand what I’m about to demonstrate, even though we may not understand the physics of it. (Even I can understand this, and you remember what I told you about me and Physics in the Introduction.) Here I am talking about vector forces to best move an object.
As I said, while the concept of “vectors” may mean nothing to you, you do understand it intuitively. Imagine yourself pulling a wagon having a year-old child in it. Picture how your body would apply the pulling force in this situation. Now imagine yourself pulling a full-grown adult in that wagon. Are you seeing that your body would “lean into” the task by lowering itself closer to the ground than the upright position you held while pulling the baby? That’s because you want to maximize the forward component of the pull over the weight (friction with the ground).
In the child example, weight (friction with the ground) really wasn’t an issue, so you simply, non-chalantly pulled the wagon without even realizing you were doing it, meaning you really applied “no” undue force”, i.e. didn’t pay attention to it, relative to simply walking, which translated into a behavior very similar to walking. Can you picture these situations?
Similarly with the bird’s sternum. The pectoral muscles (chest/breast muscles) are the flight muscles of the bird, i.e. those that have to lift the animal off the ground. They must be strong enough to do that, yet with strength, comes weight, and eventually a logistical inconsistency: As they get larger to become strong enough to lift the weight, part of which is the muscle’s weight itself, at some point their weight exceeds their strength potential. If however, the muscle could apply its force more efficiently, then it would not have to be as large (weight) to lift the same weight.
Note that the sternum of the bird has a KEEL, which our sternum doesn’t have. The keel serves two functions:
- it creates the needed “down” weight to hold the bird in proper position for flight,
- it changes the angle of attachment for the flight muscles, so that angle affords the muscle a lever advantage for the strength needed to lift the animal, while minimizing the weight of the very muscles needed to do the lifting.

Similarly with the acromion of the mole’s scapula. The mole is a digging animal. Do you notice how large its front paws are relative to other body parts? Those paws are shovels. To move them laterally, in order to shove dirt aside, the trapezius muscle attaches to this huge lever arm (as was the keel of the bird), thus giving the arm a significant advantage at moving dirt (relative to the situation were that huge acromion not there.)……..”

From: “Anatomy & Physiology: Understandable & Enjoyable, Finally!, Vol, I & II”, by Peter Karch, PhD

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