266 ANATOMY OF THE RABBIT. 
prises chiefly modified segmental muscles lying on the dorsal 
surface, for the most part in the area enclosed by the spinous and 
transverse processes of the vertebrae. They include the sacro- 
spinalis, semispinalis, and intertransversarii. Their insertions 
are extended in part laterad to the ribs. In the cervical region they 
are represented by short muscles, separated for the most part from 
the thoracic and lumbar portions, and arising by accessory bundles 
from the anterior ribs, the corresponding thoracic, and the posterior 
cervical vertebrae. In the cervical region the muscles are easily 
separated from one another, but in the posterior part of the body 
it is necessary to dissect away the tough investment of lumbodorsal 
fascia which covers them. 
(a) The sacrospinalis. Origin: Crest of the ilium and dorsal 
surface of the sacrum; mamillary processes of the six pos- 
terior lumbar vertebrae; investing lumbodorsal fascia. 
This muscle is the largest and strongest muscle of the body. 
It extends forward over the surfaces of the ribs. Its medial 
border is separated from the middle line by a space of con- 
siderable width, in which the semispinalis and multifidus 
muscles are accommodated. In the lumbar region it is 
inserted in a continuous mass on the long transverse processes 
of the vertebrae and in the interspaces. In the thoracic 
region the muscle divides into two portions, namely, a thin 
lateral portion, the iliocostalis or longissimus costarum, 
and a thick medial portion, the longissimus. The latter 
receives in the posterior portion of the thorax strong acces- 
sory bundles from the semispinalis muscle on its medial side, 
the two muscles being inseparable at this point. 
The iliocostalis is inserted laterally on the ribs as the ilio- 
costalis dorsi. It receives medially from the ribs a number 
of accessory bundles, which are inserted forwards to the 
seventh cervical vertebra as the iliocostalis cervicis. 
The longissimus is inserted by broad fleshy bands on the 
posterior margins of the ribs, medial to the accessory origins 
of the iliocostalis, this portion of the muscle forming the 
longissimus dorsi. Continuing to the neck it is inserted 
on the transverse processes of the three posterior cervical 
vertebrae, medial to the origin of the cervical portion of the 
