132 THE MUSCLES. 
lateral border of the splenius (Fig. 73, 4), and sometimes fused 
with the splenius at its craniad end.. It is a cranial continua- 
tion of the longissimus dorsi (Fig. 69, 7’). 
Origin by five slips which are attached by strong tendons 
to the anterior articular processes of the last four cervical 
vertebre. The tendons are common to this muscle and the 
complexus. The five slips unite to form a flat belly which has 
its 
Insertion by a strong rounded tendon into the mastoid 
process of the temporal bone. The tendon may be closely 
united to that of the splenius. 
Relations.—Lateral surface with the sternomastoid (Fig. 
68, c), the levator scapule (Fig. 73, 2), and longissimus dorsi 
(Fig. 69, 7’). Medial surface with the splenius (Fig. 73, 4) 
(to which it is partly united), the complexus (Fig. 69, 4), and 
the longus atlantis (Fig. 69, @). 
Action. —Lateral flexor of the head. 
M. biventer cervicis (Fig. 69, a) (medial portion of 
‘M. semispinalis cervicis et capitis).—The biventer cervicis is 
a flat muscle which lies beneath the splenius (Fig. 73, 4), next 
the median line of the neck, with its fibres longitudinal. 
Origin in three or four slips from the surface of the three 
or four aponeurotic arches which take origin from the tips of 
the spinous processes of the last cervical and the first three 
thoracic vertebrze and pass laterad and caudad to the anterior 
processes of the second, third, fourth, and fifth thoracic verte- 
bre. These arches are deviated from their straight course by 
four aponeurotic bands which pass.from them obliquely caudad 
and dorsad to the sides of the spinous processes.of the vertebrz 
into which the arches are inserted. The arches form the 
‘beginning of the sheet of fascia which passes outside of the 
longitudinal supraspinous muscles, and the anchoring bands 
form the beginning of a similar sheet which passes within the 
longitudinal supraspinous muscles. An additional slip may 
take origin from the cervical ligament between the last cervical 
and the first thoracic vertebree. The fibres form a flat band 
divided by two or three oblique tendinous intersections. It 
adheres closely to the cervical supraspinous ligament. 
