130 THE MUSCLES. 



cervicis (Fig. 69, a), and complexus (Fig. 69, [>). Lateral 

 and ventral surface with the longissimus dorsi (/') ; medial 

 surface with the spinous processes and the multifidus spinas. 



Action. — Extensor of the vertebral column. 



M. multifidus spinae. — This consists of bundles of fibres 

 which have origin on the transverse processes or neighboring 

 parts, pass craniodorsad across one or more vertebra;, and ai'e 

 inserted into the spinous processes of vertebra; lyi"S some dis- 

 tance craniad of the origin. They lie deeper than the muscles 

 previously described. The muscle is most strongly developed 

 in the lumbar region (Fig. 70, d), ^\'here it forms a thick inter- 

 woven mass in which it is difficult to distinguish separate 

 bundles. The fibres in this region have origin on the accessory 

 or mammillary processes and usually pass over more than one- 

 vertebra between origin and insertion ; their insertions reach 

 the dorsal ends of the spinous processes, so that part of the 

 muscle lies immediately beneath the lumbar fascia. In other 

 regions the multifidus is covered by other muscles. In the 

 thoracic region the separate bands are more distinct, and 

 usually pass in their course over but one intervening vertebra. 

 In the cervical region the bands are interconnected, forming a 

 fairly distinct single muscle, which is described separately 

 below as the semispinalis cervicis (Fig. 71, c). The portion 

 of this muscle attached to the head (semispinalis capitis) forms 

 the biventer cervicis (Fig. 69, a) and complexus {d). Caudad 

 this muscle passes onto the tail as the extensor caudae medialis 

 (Fig. 70, c). 



Relations. — Outer and lateral surface in the lumbar region 

 with the longissimus dorsi (h ig. 69, f) and the lumbodorsal 

 fascia (Fig. 68, y)\ in the thoracic region with the longissimus 

 dorsi (Fig. 69,/"') and spinalis dorsi {g). Inner surface with 

 the arches, articular processes, and spinous processes of the 

 vertebra;. 



Action. — Extends the back when the muscles of both sides 

 "work together. Turns the vertebral column obliquely side- 

 ways when one set acts alone. 



The deepest layer of the multifidus forms what is soirietimes 



