B. MUSCLES OF THE BACK (interconnecting parts of the vertebral column) (Fig. 69).--The muscles connected with the vertebral column form a mass which is less markedly differentiated into distinct muscles than is the case in the limbs. A great longitudinal mass of fibres begins in the sacral region and extends along the vertebral column to the head. This mass contains fibres running in various directions and attached at one or both ends to the sacrum, the innominate bones, the spinous, transverse, and articular processes, and to the arches, of the vertebræ; to the ribs and to the head, and having in general the function of moving the vertebræ in various ways, or of moving parts connected with the vertebræ. In some regions, especially the cervical, it is possible to distinguish clearly distinct 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, _f_). 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= (_h_). 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= (_g_). The separate muscles connecting the cervical vertebræ with each other and with the head are differentiations of the extensor dorsi communis. A still deeper set of fibres, interconnecting the vertebræ, forms the =multifidus spinæ= 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 interconnect the spinous processes of the vertebræ 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.