SUBOCCIPITAL MUSCLES 419 



Relations. — It lies dorso-lateral to the suboccipital muscles and to the semispinaUs cervicis. 

 From this latter it is separated by a septum containing the descending branch of the occipital 

 artery, the deep cervical artery, and the medial dorsal branches of the cervical nerves. It is 

 covered laterally by the longissimus capitis (trachelo-mastoid), and dorsaUy by the splenius, 

 and above the upper margin of the splenius by the trapezius. 



Action. — To extend the head and to incUne it slightly toward the same side. 



Variations. — The origin of the muscle may extend to the eighth thoracic vertebra or merely 

 to the first thoracic. It may be fused with the longissimus (transversalis) cervicis. A special 

 fasciculus may run beneath the muscle from the upper thoracic vertebrae to the head. The ori- 

 gin from the spinous processes of the thoracic vertebrae is not constant. The part of the muscle 

 arising from this origin may be looked upon as a spinalis capitis. 



2. Transversospinal Muscles 



The semispinalis dorsi et cervicis (fig. 383). — This superficial transverso-spinal muscle sheet 

 extends from the twehth thoracic to the second cervical vertebra. The fasciculi which compose 

 it arise by short tendons from the backs of the transverse processes, and are inserted by short 

 tendons into the spines. 



The semispinalis dorsi. — Origin. — From the sixth to the tenth or twelfth thoracic vertebrae. 



Insertion. — The upper four to six thoracic and the last two cervical vertebrae. The fas- 

 ciculi extend over four to six vertebrae. 



Nerve-supply. — Third to sixth thoracic. 



The semispinalis cervicis. — Origin. — From the upper five or six thoracic vertebrae. 



Insertion. — Into the fifth to the second cervical vertebrae. The fascicuU extend over four 

 to five vertebrae. 



Nerve-supply. — Third to sixth cervical. 



Relations. — This muscle hes beneath the longissimus dorsi and the semispinalis capitis 

 (complexus) and over the following musculature. 



Variations. — A semispinalis lumborum is a muscle rarely found extending from the lumbar 

 to the lower thoracic vertebrae. 



The multifidus (fig. 383). — This second layer of transverso-spinal musculature extends from 

 the sacrum to the second cervical vertebra. It is best developed in the lumbar region and least 

 so in the thoracic. 



Origin. — (1) Froni the groove on the back of the sacrum between the spines and the ar- 

 ticular elevations, from the dorsal sacro-iliac ligaments, from the dorsal end of the ihac crest, and 

 from the deep surface of the aponeurosis of the sacrospinal muscle; (2) from the mammary and 

 accessory processes of the lumbar vertebrae; (3) from the backs of the transverse processes of the 

 thoracic vertebrae; and (4) from the articular processes of the fourth to the seventh cervical 

 I vertebrae and the back of the transverse process of the seventh. 

 I Insertion. — Spinous processes of the lumbar, thoracic, and lower six cervical vertebrae. 



Structure. — The more superficial fasciculi arise by short tendinous processes, the deeper 

 ones directly. The more superficial fasciculi extend to the fourth or fifth vertebra above, the 

 middle to the third, and the deepest to the second above. 



The rotatores. — These, the third layer of transverso-spinal muscles, extend from the sa- 

 crum to the second cervical vertebra. They are composed of short fleshy fasciculi which ex- 

 tend to the second vertebra above {rotatores longi) and to the first" aFove {rotatores breves). The 

 fasciculi arise from the back and upper borders of the transverse processes or their homologues, 

 and are inserted into the laminae of the preceding vertebrae. They are best developed in the 

 thoracic region. Some authors consider the rotatores breves confined to the thoracic region. 

 In the cervical region the fasciculi usually run from articular processes to the bases of the spines,- 

 in the lumbar region from the mammary processes to the caudal margin of the laminae of the 

 arches. 



3. The Interspinal Muscles 



The interspinales consist of short fasciculi which extend from the upper surface of the spine 

 of each vertebra near its tip to the lower surface of the spine of the vertebra above. In the 

 neck the muscles lie in pairs between the bifid extremities of the vertebrae. In the lumbar 

 region they form broad bands attached to the whole length of the spinous processes and are 

 separated by the interspinous ligaments. In the thoracic region they usually are undeveloped. 



Nerve-supply of medial dorsal muscles. — These are all supplied by the medial branches 

 of the posterior divisions of the spinal nerves. 



Action of medial dorsal muscles. — These muscles extend the spinal column when acting on 

 both sides. When acting on one side, they produce a movement of rotation toward the opposite 

 side. 



E. Suboccipital Muscles 



(Figs. 882, 383) 



The rectus capitis posterior major. — Origin. — ^From the upper surface of the spine of the 

 epistropheus. 



Structure and insertion. — The muscle-fibres diverge to form a broad triangular band which 

 is inserted into the lateral half of the inferior nuchal line of the occipital bone and the area 

 below it. Its insertion is immediately below that of the obliquus superior. 



The rectus capitis posterior minor. — Origin. — From the upper part of the side of the posterior 

 tubercle of the atlas. 



