Introduction

In this lab, you will explore the joints and muscles of the pectoral girdle. Keep in mind that some structures are not visible using all modalities, and that is OK. By the end of the lab today, you should have identified all of the structures on the Terms to Know list using multiple sources. When using the digital atlas, use the animations to help you understand the movement of the scapulothoracic joint and the movements caused by each muscle listed.

As a study tip, you may find it helpful to write a description of each muscle/structure as you identify them. Writing it in your own words may spark your memory as you look for that structure the next time you study.

Lab Activity 1: Joints of the Pectoral Girdle and Shoulder – visible body digital atlas

Use the Visible Body digital atlas to observe the three joints of the pectoral girdle and shoulder and the structures listed with them.

• Under the Systems tab, Skeletal System Views, click on 15. Shoulder Girdle.  The “shoulder joint” is made up of three joints and has many bursae. The bursae in the app will appear purple. You can select these structures and hide them as we will not hold you responsible for knowing them. The app will also show a few more ligaments than we have listed for you. You will only be responsible for knowing the ligaments listed below.
• Examine the three joints and the structures listed with them. Highlight the structures and observe the functions listed under the book icon. Sometimes it can be helpful to attach clinical meaning to these structures when trying to remember them. You may also find it interesting to look at the injury and clinical relevance (stethoscope icon).

• • Acromioclavicular Joint
    • Acromioclavicular ligament
    • Coracoclavicular ligament (**shown as two ligaments; conoid portion and trapezoid portion. You will not need to know these two portions individually.)
  • Coracoacromial ligament
  • Glenohumeral Joint
    • Articular capsule
    • Glenoid labrum: This will be deep. Remove the capsule and other ligaments of the shoulder to see this structure.
  • Scapulothoracic joint (you cannot highlight this joint, but you can see how the scapula articulates with the ribs. You will need to zoom out or move the view medially to view this joint.)

Lab Activity 2: Muscles of the Pectoral Girdle and Shoulder – Visible Body Digital Atlas

First, explore the muscles of the pectoral girdle using the visible body digital atlas.

• Click on the Regions tab and scroll to 8. Axilla. Under the systems tab on the left side of the screen, click on the muscular system, so the whole arm is highlighted. Also, while under systems, click on the arterial, venous, nervous, and lymphatic systems twice to remove them since we will not be looking at these systems in this lab.
• Explore the muscles of the pectoral girdle and arm in the Terms to Know list. Some of these muscles will be deep to others. Thus you will have to remove the larger, more superficial muscles (pectoralis major, deltoid, trapezius, and platysma) to see the deep muscles.
• You will be asked to identify the origins, insertions, actions, and innervations of these muscles (this will be listed in the app after highlighting the structure and clicking the book). However, you may not need to know specific details for each muscle. Follow what is on the posted muscle tables for the specifics of what you need to know for each muscle.
• To see the muscles in action, click on the red pin when you have highlighted the muscle. This will drop down motion videos. You can also go back to the home screen of the app and select the Muscle Actions tab. From here, you will be able to see motions of the shoulder, and within those motion sequences, you can individually select muscles to see their action.

• • Click on the Muscle Actions tab (on the home page) and scroll to scapular motions to view the muscles that act on the scapula. Explore the animations of elevation/depression, protraction(called abduction in the digital atlas)/retraction (called adduction in the app), and upward and downward rotation of the scapula.
  • Click on shoulder motions and observe the animations of abduction/adduction, horizontal abduction/horizontal adduction, external(lateral)/internal(medial) rotation, and flexion/extension of the shoulder.

Lab Activity 3:  Navigator

Use the Navigator to examine the anatomy of the pectoral girdle. Some muscles and joint structures are visible on the 3D printed model, while others can be seen on the computer model. You should also examine the anatomy of the shoulder complex in the cross-sections.

**NOTE: DO NOT save any presets as you are examining these muscles.

• On the computer model, click on “View,” then “Advanced” in the View window to isolate specific muscles or structures. Uncheck the pectoralis major so that you can view the pectoralis minor and the subclavius. You should be able to identify most of the muscles in the list of terms, although some are cut because the model only shows the superior portion of the arm.
• Remove the muscles to examine the ligaments stabilizing the joints of the pectoral girdle.
• Observe the head of the humerus, scapula, rotator cuff muscles, and other muscles acting on the pectoral girdle and glenohumeral joint in the cross-sections.
  • Spatial relationships are key! Understanding the relationships between structures (e.g., posterior, anterior, medial, lateral, superior, inferior, superficial, deep) will help you know what you are viewing at each level. For example, with the posterior muscles acting on the pectoral girdle, you know that the trapezius is superficial (posterior) to the rhomboids.

Lab Activity 4: Upper Extremity Tissue

All muscles on the list of terms are visible either in part or on the whole on the upper extremity tissue. Be sure to view all extremity tissue available in the lab. Two arms have been dissected more superficially, while the others have been dissected so that you can identify some deeper structures.

It can be helpful for you to have the muscle tables with you while you are examining these structures. Noting the orientation of muscle fibers and the origin/insertions can help you understand the actions of these muscles.

Handle this tissue with care and respect! Be gentle when moving the specimen and feeling the muscles.

• Feel for the spine of the scapula. Extending superiorly from the scapular spine, examine part of the trapezius. This muscle has fibers running superiorly, horizontally, and inferiorly from the vertebral column to insert on the scapular spine, acromion, and clavicle. However, this muscle has been cut in some cases.
• Along the medial border of the scapula, you can identify three muscles that have been cut. From inferior to superior, these muscles are the rhomboid major, rhomboid minor, and levator scapulae. The rhomboid major is larger and inferior to the rhomboid minor. The rhomboid minor is thinner and located between the rhomboid major and the levator scapulae. The levator scapulae is the most superior of these muscles. These muscles run from the vertebral column to the medial border or superior angle of the scapula.
• Deep to the trapezius, observe the supraspinatus muscle. It has been partially retracted in the deep dissection, so you can also pull this muscle back to better appreciate how it sits in the supraspinous fossa. Be gentle when retracting the supraspinatus on the deep tissue, as there are nerves and vessels attached to the deep side of this muscle.
• Inferior to the scapular spine, observe the infraspinatus. This muscle can also be retracted in the deep dissection to observe its position in the infraspinous fossa. Again, be careful when retracting this muscle, as nerves and vessels are attached to its deep aspect.
• Just inferior to the infraspinatus, observe the teres minor. This will be best viewed on the superficial dissection. The teres minor, infraspinatus, and supraspinatus are all part of the rotator cuff muscle group and are the three rotator cuff muscles located on the posterior aspect of the scapula.
• Inferior to teres minor, observe the larger teres major. Also, observe a portion of the latissimus dorsi inferior to teres major, which is cut in this dissection. Observe these muscles on the posterior aspect of this tissue, and then turn the tissue so you can observe it from an anterior view. Now notice how the latissumus dorsi and teres major both run to insert on the anterior aspect of the humerus. This is important to appreciate to understand how these muscles contribute to the internal rotation of the glenohumeral joint.
• Observe the deltoid on the lateral aspect of the shoulder. This muscle wraps around the shoulder superiorly and comes together at a common insertion point on the humerus (the deltoid tuberosity). Notice how the orientation of the fibers changes as it wraps around the shoulder from anterior to posterior. This can help you understand why the functions of the posterior, middle, and anterior fibers of the deltoid differ.
• Observe the muscle on the anterior aspect of the scapula that has been cut but has an attachment point on the medial border of the scapula. This is serratus anterior. This muscle extends from the anterior aspect of the medial border of the scapula and wraps anteriorly around the thoracic cage to insert on the ribs. Think about how its structure contributes to its function of stabilizing the scapula.
• On the anterior aspect of the scapula, observe the subscapularis muscle. This is the fourth rotator cuff muscle, and it sits in the subscapular fossa.
• Observe pectoralis major and pectoralis minor. Both have been cut. Pectoralis major is larger and superficial to pectoralis minor. Pectorals major has an insertion on the humerus, while pectoralis minor inserts onto the coracoid process of the scapula. As a result, pectoralis major acts on the glenohumeral joint, while pectoralis minor only acts on the scapulothoracic joint.
• Notice a small amount of muscle that is located on the inferior aspect of the clavicle. This muscle has been cut, but you can see the superior most portion of the subclavius.

Now you will examine muscles of the arm that cross the glenohumeral joint.

• Observe the coracobrachialis. This muscle originates on the coracoid process of the scapula and inserts on the humerus. This muscle is typically pierced by a nerve, the musculocutaneous nerve, in the middle of the muscle belly. However, notice how it differs in one of the deep dissections. In this case, an earlier portion of the brachial plexus also pierces this muscle, giving it the appearance of having two muscle bellies. Anatomical variation is very common! 
• Now observe the biceps brachii. This muscle has two heads. The long head originates on the supraglenoid tubercle of the scapula. Follow it as high as you can on the deep dissection, towards the head of the humerus. The short head originates on the coracoid process of the scapula.
• Just lateral to the biceps brachii, observe the brachialis. This muscle is the primary flexor of the elbow.
• On the posterior aspect of the arm, observe the triceps brachii. The long head is superficial and in the middle, and this is the only portion of the triceps brachii that crosses the glenohumeral joint. Also, observe the medial and lateral heads.

Examine the joints of the pectoral girdle:

• Observe the acromioclavicular joint on these dissections. You can palpate where the clavicle meets the acromion. On the superficial dissection, you may be able to see the acromioclavicular and coracoclavicular ligaments.
• On the deep dissection, you can also see the coracoacromial ligament running from the acromion process to the coracoid process of the scapula. Observe how the supraspinatus runs underneath this ligament as it moves towards the greater tubercle.
• Finally, look at the posterior aspect of the deep dissection. The infraspinatus has been cut, and the articular capsule has been opened so that you can observe the internal aspect of the glenohumeral joint. Notice how smooth the articular cartilage on the head of the humerus is. Also, note how thick the articular capsule is. Observe the ring of tissue just surrounding the glenoid fossa. This is the glenoid labrum.

Be sure to examine the structures above on the plastinated tissue as well. Not all muscles are visible in this dissection. (Note: The vessels and nerves on this specimen are very fragile. Please be gentle!)