MUSCLES OF THE BACK 417 



Variations of the sacro-spinalis. — The slips of origin and insertion of the various parts of 

 •this muscle and the extent of fusion of the various parts vary greatly. Statistical data from 

 which the most frequent conditions might be determined are wanting. Tendinous inscriptions 

 may extend across the longissimus cervicis and other parts of the sacro-spinalis. 



B. Deep Lateral Dorsal System 



The intertransversarii. — These are vertical bands composed of short bundles which pass 

 between the transverse processes of the cervical, lumbar, and the lower thoracic vertebrae. 



(a) Cervical (fig. 349) . — Ventral, lateral and dorsal muscles are found in the cervical region. 

 The ventraTatnd lateral muscles run between the ventral tubercles and tips of the transverse 

 processes of the vertebrse, are homologous with the intercostal muscles, are supplied by branches 

 from the anterior divisions of the corresponding spinal nerves, and have been described above 

 (p. 356). The dorsal muscles run between the dorsal tubercles and belong to the intrinsic 

 dorsal musculature. They are supphed by the lateral branches of the posterior divisions of the 

 cervical nerves.* The three sets of muscles are, however, more or less fused. The first pair 

 of muscles extends between the atlas and axis, the lowest passes to the transverse process of the 

 first thoracic vertebra, or to the first rib close to this. The obliquus capitis superior (described 

 later) belongs, however, to the posterior set of muscles, the rectus capitis lateralis (p. 356) to 

 the lateral set. The vertebral artery runs vertically betw^een each pair of muscles above the 

 sixth, and the anterior division of each cervical nerve passes laterally between the artery and the 

 dorsal muscle in each space, and then out between the ventral and lateral muscles. The pos- 

 terior division of each cervical nerve passes medial to each dorsal muscle. 



(b) Thoracic. — Small muscle fasciculi may extend between the transverse processes of the 

 thoracic vertebrae and between the last thoracic and first lumbar. They are most frequent in 

 the upper and lower thoracic regions. Often they are replaced by tendinous bands. In the 

 second interspace the insertion maj^ extend to the rib near the transverse process. The inner- 

 vation is from the lateral branches of the posterior divisions of the spinal nerves. 



(c) Lumbar,{iig. 383). — In the lumbar region there is a lateral set of muscles connecting 

 the adjacent margins of the transverse processes and a medial connecting the mammary tubercle 

 of one vertebra to the mammary or the accessory tubercle of the vertebra next above. They 

 extend between each two of the five lumbar vertebrae and sometimes also to the first sacral. 

 They lie between the sacrospinalis and psoas muscles. The medial muscles are supplied by 

 the lateral branches of the posterior divisions of the spinal nerves. The lateral muscles are 

 supplied by branches from the junction between the two divisions of the corresponding spinal 

 nerves. These branches probably belong to the anterior divisions. 



Action. — The intertransverse muscles bend the spinal column laterally, and when acting 

 on both sides, make it rigid. 



Variations. — The number of intertransverse spaces occupied by the muscles varies, espe- 

 cially in the thoracic region. They may be doubled or extend over more than one interspace. 



C. Superficial Medial Dorsal System 



The spinalis dorsi. — Origin. — By tendinous bands from the tips of the two upper lumbar 

 and the last two thoracic spines. 



Structure and insertion. — From the deep surface of the tendinous bands there arises a long 

 slender muscle belly which is fused laterally with the longissimus dorsi. It is attached by 

 tendinous processes to the spines of the upper thoracic vertebrae, usually the second or third to 

 the ninth. 



Nerve-supply. — From the medial divisions of the sixth to ninth thoracic nerves. 



The spinalis cervicis. — A muscle of inconstant development which arises from the spines 

 of the two upper thoracic and two lower cervical vertebrae and extends to the spines of the second 

 to the fourth cervical vertebrae. The nerve supply is from the dorsal divisions of the lower cer- 

 vical nerves. 



Action. — To extend the vertebral column. 



Variation. — There is great variation in the development of the spinalis muscles. Similar 

 f ascicuh are sometimes found in the lumbar region and in the cervical region sometimes extend 

 to the skull. 



D. Deep Medial Dorsal System 



1. Vertehro-occipital Muscle 



The semispinalis capitis (complexus) (fig. 381). — This muscle is usually separated from the 

 semispinalis muscles of the back and neck by a well-marked septum and has a distinctive 

 structure. 



Origin. — (1) By long tendinous fasciculi from the tips of the transverse processes of the 

 upper five or six thoracic vertebrae and of the seventh cervical vertebra; (2) by short fleshy 

 processes from the articular processes and bases of the transverse processes of the third to the 

 sixth cervical vertebrae; and (3) by deUcate fleshy f ascicuh from the spinous processes of the 

 upper thoracic vertebrae. 



Structure and insertion. — The slightly diverging fibre-bundles form a long, flat beUy which 

 is inserted, partly by means of an aponeurosis which covers the muscle laterally, into the lower 



* According to Lickley, both sets of cervical intertransverse muscles are suppUed by the 

 anterior divisions of the spinal nerves. 



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