124 THE MUSCLES. 
which have been differentiated from this mass. In the greater 
part of its extent, however, it is possible to distinguish only 
more or less closely interconnected bundles,—the more clearly 
marked of which have received special names. 
The great dorsal mass running along each side of the 
vertebral column, occupying the space between the spinous 
processes and the transverse processes, receives the general 
name of M. extensor dorsi communis; the separate muscles 
are to be considered as parts or differentiations of this. The 
largest part of the mass is in the lumbar region (first portion 
of the longissimus dorsi, Fig. 69, 7). At the caudal end of 
the thorax a lateral mass, parts of which become connected 
with the ribs, is separated off from the main part of the 
muscle; this extends craniad into the cervical region as 
M. iliocostalis (4). The main portion of the longissimus dorsi 
(f') continues craniad on the medial side of the iliocostal into 
the neck region. At about the level of the eighth or ninth 
thoracic vertebra a strip begins to become separated off on the 
medial side of the longissimus; further forward it becomes 
clearly distinct, forming the spinalis dorsi (zg). The separate 
muscles connecting the cervical vertebrz with each other and 
with the head are differentiations of the extensor dorsi com- 
munis. 
A still deeper set of fibres, interconnecting the vertebre, 
forms the multifidus spinz and a number of other small 
muscles. . 
A description of the different bundles of fibres, under the 
names usually given, follows. Certain general principles are 
observed in some of the names used. Muscles which intercon- 
nect the spinous processes of the vertebrz receive the name 
spinalis.. Those interconnecting contiguous spinous processes 
are called interspinalis. Muscles attached at one end to 
transverse processes, at the other to the spinous processes, 
receive the name transversospinalis. Semispinalis has the 
same signification as transversospinalis, but is a name usually 
applied to subdivisions of the transversospinalis group. The 
intertransversarii are muscles interconnecting the transverse 
processes. 
