Lab 3 Pre-Lab Exercise:
1. Surface anatomy of the back
Surface features of the back are used to locate muscle groups, identify major bony landmarks, determine regions of the vertebral column, and to approximate the position of the spinal cord. These features are also used to locate organs that occur posteriorly in the thorax and abdomen. With the help of your teammates and using the image below, find the following surface anatomy landmarks of the back.
The spinal cord does not occupy the entire length of the vertebral column. In adults, it ends around the level of the disc between vertebrae L1-L2. The subarachnoid space ends at approximately the level of the second sacral vertebra. This makes the subarachnoid space accessible in the lower lumbar region without endangering the spinal cord. It is therefore important to be able to identify the spinous processes of this region.
The L4 spinous process is at the same level as the highest point of the iliac crests. In the lumbar region, the spinous processes lie opposite of their corresponding vertebral bodies. Therefore, the subarachnoid space can be accessed between the L3-L4 or between L4-L5 vertebral levels (above and below the spinous process of L4). Reminder that the subarachnoid space ends at the S2 vertebral level, located at the level of the posterior superior iliac spines (sacral depressions).
2. Surface anatomy of the ventral body wall
The ability to visualize anatomical structures in the thorax in relation to the surface features is fundamental to a physical exam. With the help of your teammates and using the image below, find the following surface anatomy landmarks of the ventral body wall.
3. Identifying bony landmarks
Sternum: The jugular (suprasternal) notch lies between the clavicles and the upper border of the manubrium, at the level of the lower border of the body of T2. The manubrium lies at the same level as the bodies of T3 and T4 (in front of the arch of the aorta). The sternal angle is felt about 4-5cm inferior to the jugular notch (represents the junction of the manubrium and body of the sternum - manubriosternal joint), lies at the level of the 2nd costal cartilage (or lower body of T4). The body of the sternum is about 10cm long and lies in front of the T5-T8 vertebral bodies (as well as in front of the heart). The xiphisternal joint is at the level of the T9 body.
Clavicles: Can be felt from end to end (subcutaneous) since they produce sinuous horizontal ridges visible at the junction of the neck to the thorax. Each clavicle is “S” shaped with its medial 2/3 convex anteriorly and its lateral 1/3 concave anteriorly. Palpate the acromioclavicular (with acromion of scapula) and sternoclavicular (sternum) joints.
Knowing how to count ribs is important because different ribs provide palpable landmarks for the positions of deeper structures.
Ribs: 2nd-12th can usually be palpated.
To number the ribs, start anteriorly with the 2nd costal cartilage (to the 2nd rib) at the sternal angle and count in an infero-lateral direction. The intercostal spaces are numbered from one, which is located between the 1st and 2nd rib.
Posteriorly, the medial end of the spine of the scapula overlies the 4th rib, the 8th rib lies just below the inferior angle of the scapula, the 11th rib is the lowest rib crossed by the margin of latissimus dorsi.
Costal cartilages: The 1st cartilage (about 2.5cm long) lies below the medial end of the clavicle. The 7th cartilage is to articulate directly with the sternum. The 10th cartilage forms the lowest part of the costal margins when viewed anteriorly, lying at the level of the L3 body.
4. Identifying major muscles
Pectoralis major: Can be seen and felt throughout its entire extent when it is contracted against resistance as in pressing the palms together in front of the body (or as seen in the image above). Pectoralis major has two parts: 1) clavicular fibers can be felt if the shoulder is flexed against resistance to a position midway between flexion and extension, and 2) sternocostal fibers can be felt if the shoulder is extended against resistance starting in a flexed position.
Serratus anterior muscle: Its digitations of origins may be seen on resisted protraction of the scapula.
Anterior axillary fold: formed by the inferior border of the pectoralis major muscle.
Posterior axillary fold: formed by the latissimus dorsi muscle winding around the lateral border of the teres major muscle.
Deltopectoral triangle (infraclavicular fossa): Bounded by the clavicle superiorly, the deltoid muscle laterally, and the clavicular head of the pectoralis major muscle medially.
5. Surface anatomy of the breast in women
Though breasts vary in size, they are usually positioned on the thoracic wall between ribs 2 and 6 and overlie the pectoralis major muscles. Each mammary gland extends supero-laterally around the lower margin of the pectoralis major muscle and enters the axilla. This portion is the axillary tail or process. The position of the nipple and areola vary relative to the chest wall depending on breast size.