130 THE MUSCLES, 
cervicis (Fig. 69, a), and complexus (Fig. 69, 4). Lateral 
and ventral surface with the longissimus dorsi (/’); medial 
surface with the spinous processes and the multifidus spine. 
Action. —Extensor of the vertebral column. 
M. multifidus spinz.—This consists of bundles of fibres 
which have origin on the transverse processes or neighboring 
parts, pass craniodorsad across one or more vertebrz, and are 
inserted into the spinous processes of vertebre lying 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, @), where 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 (6). Caudad 
this muscle passes onto the tail as the extensor caude medialis 
(Fig. 70, e). 
Relations.—Outer and lateral surface in the lumbar region 
with the longissimus dorsi (Fig. 69, /) and the lumbodorsal 
fascia (Fig. 68, y); in the thoracic region with the longissimus 
dorsi (Fig. 69, 7’) 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 sometimes 
