148 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 238. — The deeper layers of the flat muscles of the back. 



On the left side the trapezius and latissimus have been cut away; on the right side the rhomboidei have also been 

 cut and reflected and the lumbodorsal fascia has been retained only where it is in relation with the origin of the 

 serratus posterior inferior and below. 



The Third Layer. 



The muscles of this layer (Figs. 238 and 239) are situated partly beneath the rhomboidei and 

 partly directly beneath the latissimus and the trapezius. The two splenii are also covered at their 

 origins by a muscle of the same layer, the serratus posterior superior. 



The serratus posterior superior (Fig. 238) is a flat muscle, rhomboidal in shape, which 

 is tendinous for almost half its breadth and is wholly or almost wholly covered by the rhomboidei. 

 It arises by an aponeurosis from the spinous processes of the two lowermost cervical and of the 

 two uppermost thoracic vertebras, passes outward and downward, and is inserted by four flat 

 muscular digitations into the outer side of the angles of the second to the fifth ribs. 



It is supplied by the upper (first to fourth) intercostal nerves. It draws the upper ribs upward and backward and 

 acts as a muscle of inspiration by enlarging the thorax. 



The serratus posterior inferior (Fig. 238) resembles the serratus posterior superior in 

 many respects, but it is flatter and its fibers run from within outward and from below upward. 

 It arises from the anterior surface of the posterior layer of the lumbodorsal fascia in common with 

 the latissimus dorsi, at the level of the two lowermost thoracic and the two uppermost lumbar 

 vertebrae; it is at first a thin and independent aponeurosis, but subsequently becomes entirely 

 muscular, and is inserted into the lower borders of the lower four ribs by digitations which fre- 

 quently vary in their development or may be wanting. 



The muscle is supplied by the lower (ninth to twelfth) intercostal nerves. It draws the lower ribs backward and 

 downward. Whether it aids inspiration or expiration is uncertain; in either case its influence upon the ribs is very 

 slight. It may increase the tension of the lumbar fascia. 



The splenius capitis (Figs. 238 and 239) is a strong, elongated, strap-shaped muscle, which 

 arises bv me ans of the nuchal ligament from the spinous processes of the lower four or five cervical 

 and from the upper two or three thoracic vertebras. It passes from within outward and from below 

 upward, partly covered by the trapezius, the rhomboidei, and the serratus posterior superior, and 

 its insertion is into the outer half of the uppermost nuchal line of the occipital bone extending as 

 far as the mastoid process. The insertion is covered by the sternocleidomastoid. 



The sp lenius cerv icis (Fig. 239) is situated immediately to the outer side of the splenius 

 capitis. It arises in immediate succession to the latter muscle from the spinous processes of the 

 third or fourth to the fifth or sixth thoracic vertebras, and passes obliquely outward and upward 

 as a flat but rather slender muscle, to be inserted into the posterior tubercles of the tra nsverse 

 processes of the upper two or three cervical vertebrae. Its insertion is intimately connected with 

 the origin of the levator scapulas. 



The two splenii are supplied from the posterior divisions of the second to the eighth cervical nerves. When the 

 muscles of both sides act in common, they pull the head (or the neck) backward, and when the muscles of one side 

 act alone, they turn the head (or the neck) toward the side of the contracting muscle. 



