Muscles of the Back

We now consider muscles of the back that extend, rotate, and abduct the vertebral column (figs. 10.17-10.19). Back muscles that act on the pectoral girdle and arm are considered later. The muscles associated with the vertebral column moderate your motion when you bend forward and contract to return the trunk to the erect position. They are classified into two groups—a superficial group, which extends from the vertebrae to the ribs, and a deep group, which connects the vertebrae to each other.

In the superficial group, the prime mover of spinal extension is the erector spinae. You use this muscle to maintain your posture and to stand up straight after bending at the waist. It is divided into three "columns"—the iliocostalis, longissimus, and spinalis. These are complex, multipart muscles with cervical, thoracic, and lumbar portions. Some portions move the head and have already been discussed, while those that act on cervical and lower parts of the vertebral column are described in table 10.7. Most of the lower back (lumbar) muscles are in the longissimus group. Two serratus posterior muscles—one superior and one inferior—overlie the erector spinae and act to move the ribs.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Abdominal Oblique

Pectoralis minor

Serratus anterior

Rectus sheath -

Internal abdominal -oblique

Internal abdominal -oblique

Rectus Abdominis Muscle

Subclavius Pectoralis minor (cut)

Internal intercostals

External intercostals Rectus abdominis (cut)

External abdominal oblique (cut)

Internal abdominal oblique (cut)

Transversus abdominis (cut)

Figure 10.15 Thoracic and Abdominal Muscles. (a) Superficial muscles. The left rectus sheath is cut away to expose the rectus abdominis muscle. (b) Deep muscles. On the anatomical right, the external abdominal oblique has been removed to expose the internal abdominal oblique and the pectoralis major has been removed to expose the pectoralis minor. On the anatomical left, the internal abdominal oblique has been cut to expose the transversus abdominis, and the rectus abdominis has been cut to expose the posterior rectus sheath.

Subclavius Pectoralis minor (cut)

Internal intercostals

External intercostals Rectus abdominis (cut)

External abdominal oblique (cut)

Internal abdominal oblique (cut)

Transversus abdominis (cut)

Posterior wall of rectus sheath (rectus abdominis removed)

Figure 10.15 Thoracic and Abdominal Muscles. (a) Superficial muscles. The left rectus sheath is cut away to expose the rectus abdominis muscle. (b) Deep muscles. On the anatomical right, the external abdominal oblique has been removed to expose the internal abdominal oblique and the pectoralis major has been removed to expose the pectoralis minor. On the anatomical left, the internal abdominal oblique has been cut to expose the transversus abdominis, and the rectus abdominis has been cut to expose the posterior rectus sheath.

Saladin: Anatomy & I 10. The Muscular System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 10 The Muscular System 349

Pectoralis major

Pectoralis major

External abdominal oblique

Tendinous intersection Linea alba

Rectus abdominis

Rectus sheath Internal abdominal oblique

Rectus Abdominis Function
4

Figure 10.16 Thoracic and Abdominal Muscles of the Cadaver.

The major deep thoracic muscle is the semi-spinalis. This is divided into three parts, the semi-spinalis capitis, which we have already studied (see table 10.4), the semispinalis cervicis,40 and semispinalis thoracis,41 in that order from superior to inferior. In the lumbar region, the major deep muscle is the quadratus42 lumborum. The erector spinae and quadratus lumborum are enclosed in a fibrous sheath called the thoracolum-bar fascia, which is the origin of some of the abdominal and lumbar muscles. The multifidus43 muscle deep to

40cervicis = of the neck

41 thoracis = of the thorax

42quadrat = four-sided

43multi = many + fid = split, sectioned this connects the vertebrae to each other from the cervical to the lumbar region and acts to extend and rotate the vertebral column.

Insight 10.2 Clinical Application

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