Chapter 8: The Appendicular Skeleton

Transcription

1 Chapter 8: The Appendicular Skeleton In Chapter 8, we complete our tour of the skeleton that began in Chapter 7. As with Chapter 7, much of this material is best learned in lab, but we will outline some key points here for context and reinforcement. Chapter 9 then looks at how bones come together to form joints. 8.0: Outline 8.1: What s included in the appendicular skeleton? The appendicular skeleton includes the pectoral and pelvic girdles and the upper and lower limbs. 8.2: The pelvis: a fusion of confusion The pelvis includes the pelvic girdle plus the sacrum and coccyx. Each bones of the pelvic girdle is a fusion of the ilium, ischium, and pubis. 8.3: Bone markings revisited Recall that bone markings generally enable formation of joints, attachment of tendons and ligaments, or accommodation of nerves or blood vessels. Examples of prominent bone markings include the femur s head, which forms a joint with the acetabulum of the pelvis, and the femur s greater trochanter, where tendons from several muscles attach. 8.4: The effect of sex and age on bone structure The pelvis of females tends to be broader (for carrying a fetus) and to have a wider pelvic inlet and pelvic outlet (for delivering a fetus). Sex difference stem from the TDF gene on the Y chromosome, which causes gonads to develop into testes, which produce testosterone. Bones continue to ossify and fuse with other bones throughout childhood. Reductions in bone mass are common later in life, especially after age : Recommended review questions 1

2 8.1: What s included in the appendicular skeleton? As we saw at the start of Chapter 7, appendicular skeleton is appended (attached) to the axial skeleton. Its 126 bones can be grouped as follows. Pectoral girdles (4) o Clavicle ( collarbone ) 2 o Scapula ( shoulder blade ) 2 Upper limbs (60) o Humerus 2 o Ulna 2 o Radius 2 o Carpals 16 o Metacarpals 10 o Phalanges 28 Pelvic girdle (2) o Hip bone 2 Lower limbs (60) o Femur 2 o Patella 2 o Tibia ( shin bone ) 2 o Fibula 2 o Tarsals 14 includes calcaneus (heel bone) and talus (forms joint with tibia) o Metatarsals 10 o Phalanges 28 A visual overview of the appendicular skeleton is given by 10 th Martini Figure 8-1 (An Anterior View of the Appendicular Skeleton). The outline above includes the so-called pectoral girdles and pelvic girdle. These terms may not be intuitive to everyone. In general, a girdle can be defined as an encircling or ringlike structure (rhymezone.com), often providing structural reinforcement. Here, the pectoral and pelvic girdles essentially surround and reinforce the connections between the axial skeleton and the limbs. Note that, in the table above, the numbers of bones listed cover the left and right sides summed together. While the totals aren t especially important, it is interesting that the upper limbs and lower limbs each contain 60 bones. Furthermore, moving from proximal to distal each upper limb and each lower limb have a single long bone closest to the axial skeleton, then two long bones side by side, then 7 or 8 carpals or tarsals, 5 metacarpals or metatarsals, and 14 phalanges. Although the lower limb has fewer tarsals (7) than the upper limb has carpals (8), the lower limb also has an additional bone the patella, or kneecap so the total number of bones for the lower limb is the same as for the upper limb. You may wonder why 10 th Martini refers to upper limb and lower limb rather than, say, arm and leg. To an anatomist, the first pair of terms is not equivalent to the second pair; the 2

3 arm is defined as the shoulder to the elbow, and the leg is defined as the knee to the ankle. (The region from the elbow to the wrist is the forearm, and the region from the hip to the knee is the thigh.) Thus, upper limb and lower limb are more inclusive terms that cover the entire limb, including the hand or foot. 8.2: The pelvis: a fusion of confusion When we classify each bone as belonging to the axial skeleton or the appendicular skeleton, we must be careful when we get to the pelvis. What s the issue? an attentive student may ask. The outline above clearly indicates that the pelvic girdle is appendicular! Yes, but pelvic girdle is not quite the same as pelvis. The pelvis is defined as the pelvic girdle PLUS the sacrum and coccyx, which are part of the vertebral column and therefore part of the axial skeleton (see Chapter 7). Returning to the pelvic girdle, we see from the outline above that it consists of two bones. Various sources refer to these as the hip bones, pelvic bones, coxal bones (remember coxal from Exercise 1 in your lab manual?), or os coxae. Each of the two bones is itself a fusion of three bones: the ilium, the ischium, and the pubis. Consult 10 th Martini Figure 8-7 (The Right Hip Bone), noting that the ilium is superior to the ischium and pubis and that the pubis is anterior to the ischium. You should not be obsessed with all of this terminology, but you should not let it confuse you either! 10 th Martini Figure 8-7 shows a hip bone in isolation, but we should remember this bone s connections to other bones, which are much clearer in 10 th Martini Figure 8-8 (The Pelvis of an Adult Male). In the posterior, the hip bone joins the sacrum; in the anterior, the pubis joins the pubis from the other side at the pubic symphysis, which is made out of fibrocartilage. 8.3: Bone markings revisited In Chapter 6, we saw that bones can have many different types of features or markings: canals, condyles, crests, etc. We said that there are three main roles for many of these markings: forming joints, providing attachment points for tendons or ligaments, and (for depressions and openings) accommodating blood vessels and nerves. Now that we are looking at the skeleton in some detail, we can start to notice and appreciate examples of these features. In the skull (Chapter 7), there were many obvious depressions and openings (fissures, foramina, etc.) for cranial nerves and arteries. The appendicular skeleton provides some especially dramatic examples of joint-forming and attachment-facilitating projections. Take the femur, for example. The head of the femur as pictured in 10 th Martini Figure 8-11 (Bone Markings on the Right Femur) is a large expansion of the femoral neck into a dome-like structure. Note how 3

4 well this fits into the acetabulum of the pelvis (10 th Martini Figure 8-7). The resulting joint is much more stable than if the femur was simply a stick projecting to a flat surface on the pelvis. Another prominent feature of the femur is the greater trochanter (10 th Martini Figure 8-11). It is not part of a joint, so why is it there? It turns out that several muscles originating in the pelvis attach at the greater trochanter. As shown in 10 th Martini Table (Muscles That Move the Thigh) and Figure (Muscles That Move the Thigh), these muscles are as follows: gluteus medius, gluteus minimus, obturator internus, piriformis, gemelli. Find these muscles in Table 11-16; note that each has an origin somewhere in the pelvis and an insertion onto the greater trochanter. Because the greater trochanter expands the area of bone available for attachment, these muscles are bigger and stronger than they could be otherwise. 8.4: The effect of sex and age on bone structure If we compare the skeletons of a typical female and a typical male, we notice many fairly subtle differences. These are summarized in 10 th Martini Figure 8-10 (Sex Differences in the Human Skeleton). In general, males tend to have larger, heavier bones with more prominent bone markings. A comparison of the female and male pelvi is especially interesting, as the female pelvis includes adaptations for carrying and delivering fetuses. As seen in 10 th Martini Figure 8-10, the ilia (plural of ilium) of the hip bones form a kind of bowl. This bowl is usually wider in females so that there is more room for a fetus to grow. Women are thus said to have wide hips sometimes described as child-bearing hips, which is anatomically correct but arguably offensive in its implication that having children should be a priority for women. The pelvis also includes both a pelvic inlet and a pelvic outlet through which the fetus must pass during childbirth, so both inlet and outlet tend to be larger in the female. The inlet tends to be described as heart-shaped in the male and more circular in the female. The coccyx (pronounced COCK-six ) impinges on the outlet see 10 th Martini Figure 8-9a (Divisions of the Pelvis) but less so in the female, where the coccyx points more inferiorly (downward) and less anteriorly. As physiologists, we should not only ask HOW male and female skeletons differ, but WHY they differ. While the full story of sex-specific development is beyond the scope of this course, CTM Figure 8.1 provides a partial explanation. The Y chromosome contains a gene for a protein called the Testis Determining Factor (TDF), which binds to DNA and alters the expression (transcription and translation into protein) of various other genes are expressed. The ultimate result is that gonads lacking the TDF gene develop into ovaries, while gonads with this gene for TDF develop into testes, which produce testosterone, which, over many years, contributes greatly to sex differences such as differences in bone structure. Chapter 8 of 10 th Martini does not say much about how aging affects the skeleton, but does offer Table 8-1 (Age-Related Changes in the Skeleton), which is a nice compilation covering changes in both children and adults. Check it out. The bottom line is that ossification (bone formation) 4

5 and fusion of adjacent bones occur throughout childhood, while reductions in bone tissue are common later in life, especially after age 60. CTM Figure 8.1: The origin of sex differences. Image taken from Genetics of Sexual Differentiation by Harry Ostrer (glowm.com/resources/glowm/cd/pages/v5/v5c078.html). 8.5: Recommended review questions If your understanding of this chapter is good, you should be able to answer the following 10 th Martini questions at the end of Chapter 8: #3, #4, #6, #9, #13, #15, #17, #18, #19, #21, and #23. (Note that these are NOT the Checkpoint questions sprinkled throughout the chapter.) Explanation This file is my distillation of a chapter in the textbook Fundamentals of Anatomy & Physiology, Tenth Edition, by Frederic H. Martini et al. (a.k.a. the 10 th Martini ), and associated slides prepared by Lee Ann Frederick. While this textbook is a valuable resource, I believe that it is too dense to be read successfully by many undergraduate students. I offer Crowther s Tenth Martini so that students who have purchased the textbook may benefit more fully from it. No copyright infringement is intended. -- Greg Crowther 5