WHAT IS THE APPENDICULAR SKELETON? - MEDICINE - 2024

The appendicular skeleton is the set of bones that make up the mobile part of the human skeleton. Made up of 206 bones, the human skeleton is divided into two parts, an axial or central skeleton made up of 80 bones that make up the head, thorax, and spine, and an appendicular skeleton made up of the upper and lower extremities.

The function of the axial skeleton is to serve as the central axis of the body and to protect the vital organs, as well as to serve as a surface for the insertion of muscles that allow the mobility of the appendicular skeleton attached to it.

For its part, the axial skeleton is made up of the 126 remaining bones that articulate with the central body axis or axial skeleton, to form the upper and lower extremities. In this way, one of the characteristics of the appendicular skeleton is that each of the bones that make it up are bilateral.

It gets its name from the word "appendix", which comes from the verb pendere, which means "to hang", that is to say, a structure that develops from its attachment to a base or central structure.

How is it made?

The appendicular skeleton is made up of 4 bones that make up the shoulder girdles, 60 bones for the upper limbs, 2 bones that make up the pelvic girdle, and 60 bones that make up the lower limbs.

The pelvis is considered as a single bone structure, but it must be taken into account that the ilium, ischium and pubis join to form a single bone; the innominate bone.

The hip bone articulates with the sacrum posteriorly and with the contralateral hip bone anteriorly. By means of the amphiarthrosis denominated "symphysis pubis" they form the pelvis.

How does the appendicular skeleton articulate with the axial skeleton?

The shoulder girdle and the pelvic girdle are the structures that join the upper and lower limbs to the axial skeleton respectively.

Superior limbs

The only joint that truly connects the shoulder girdle to the axial skeleton is the sternoclavicular joint, also called the sternocostoclavicular joint.

This joint joins the sternum, clavicle and the first costal cartilage, by means of joint capsules, synovials and ligaments that strengthen the joint and limit its mobility.

The rest of the means of attachment are made up of muscles whose origin and insertion are from the upper limb or the shoulder girdle (appendicular skeleton) to the rib cage or vertebral column (axial skeleton) and vice versa.

These muscles are from superficial to deep the following:

• Trapezius: its axial attachments go in the superior nuchal line and external occipital protuberance above, the posterior cervical ligament inside, and in the spinous processes of the 7th cervical vertebra to the 11th thoracic vertebra below. Its appendicular attachments go to the clavicle and scapula.
• Rhomboid major: has its origin in the spinous processes of the thoracic vertebrae 2, 3, 4 and 5 and in the supraspinatus ligament, and is attached to the spine of the scapula.
• Pectoralis major: consists of 3 parts, and only its abdominal part is part of the union of both skeletons.

The fibers that make up the abdominal part of the pectoralis major originate from the cartilages of the 7th, 8th, and 9th rib, and attach to the crest of the greater tubercle of the humerus.

• Pectoralis minor: it has three bundles of fibers that originate from the 3rd, 4th and 5th ribs, and insert into the coracoid process of the scapula.
• Serratus anterior: it has three origins distributed between ribs 1 to 6, and they insert in the superior angle, medial border and inferior angle of the scapula.
• Subclavian: it has its origin in the first costal cartilage and its insertion in the clavicle.
• Scapulae levator: its fibers originate in the transverse processes of the cervical vertebrae 1, 2, 3 and 4, and insert into the spine of the scapula.

Lower extremities

The pelvic girdle, for its part, articulates with the axial skeleton through the sacroiliac joint, which joins the coxal bone to the spine through dense and strong ligaments.

These ligaments have the function of securing the joint to transmit the weight of the spine to the lower limbs.

The ligaments responsible for stability are:

• anterior sacroiliac.
• posterior sacroilliac.
• iliolumbar.
• sacrociatics.
• sacrospinous.
• sacrotuberous.

Although some muscles also connect the lower extremities to the spine, their functions are to give some range of motion to the lower limb or to form the pelvic floor. The result is to support and fix the pelvic organs, and not to properly unite the lower limb to the central axis of the body, as in the case of the upper limbs.

References

1. Atlas of Human Anatomy. Frank H. Netter, MD 3rd Edition. Editorial Elsevier. Barcelona - Spain (2003). Sheets 340-341, 406-407, 468-469.
2. Human Anatomy Alfredo Latarjet Ruiz Liard. Editorial Médica Panamericana. (2004) Volume 1.
3. Visible body. Hips, Shoulders, Arms, and Legs: Bones of the Appendicular Skeleton. Recovered from: visiblebody.com
4. Spine Health Knowledge from veritas. Sacroiliac Joint Anatomy. By Peter F. Ulrich, MD Orthopedic Surgeon. Updated: 11/10/2010. Recovered from: spine-health.com
5. Teach Me Anatomy. The Sternoclavicular Joint. Ron Sangal December 27, 2017. Recovered from: teachmeanatomy.info