and 



THE MUSCLES OF THE BACK. 439 



upper three or four thoracic vertebrae. It is directed obliquely downwards 

 and laterally, to be inserted, by separate slips, into the second, third, fourth, and 

 fifth ribs. The muscle is -concealed by the vertebro-scapular muscles, and crosses 

 obliquely over the splenius, sacrospinalis and semispinalis capitis. It lies super- 

 ficial to the lumbo-dorsal fascia. 



Nerve-Supply. Posterior rami of upper thoracic nerves. 



Actions. It is an accessory muscle of inspiration and an extensor of the vertebral column. 

 Acting on the vertebral column, from the costal attachment, it assists in lateral movement of the 



column. 



M. Serratus Posterior Inferior. The serratus posterior inferior has a 

 membranous origin, through the medium of the lumbo-dorsal fascia, from the last 

 two thoracic and first two lumbar spinous processes. 



It forms four muscular bands which pass almost horizontally to an insertion 

 into the last four ribs. The muscular slips overlap one another from below 

 upwards. The muscle is on the same plane as the posterior layer of the lumbo- 

 dorsal fascia, and is concealed by the latissimus dorsi. 



Nerve-Supply. Posterior rami of the lower thoracic nerves. 



Actions. The muscle is an extensor of the vertebral column and an accessory muscle of 

 inspiration, raising, everting, and fixing the lower four ribs. 



M. Splenius. The splenius muscle is a broad, flattened band which occupies 

 the back of the neck and the upper part of the thoracic region. It arises from the 

 ligamentum nuchae (from the level of the fourth cervical vertebra downwards) and 

 from the spinous processes of the last cervical and higher (four to six) thoracic 

 vertebras. 



Its fibres extend upwards and laterally into the neck, separating in their course 

 into an upper and a lower part. The upper part forms the splenius capitis, 

 which is inserted into the rnastoid portion of the temporal bone and the lateral 

 part of the superior nuchal line of the occipital bone (Fig. 396, p. 444). The 

 lower part forms the splenius cervicis, which is inserted into the posterior 

 tubercles of the transverse processes of the upper three or four cervical vertebrae, 

 behind the origin of the levator scapulae. 



The muscle is partially concealed by the trapezius and sterno-mastoid, and 

 appears between them in the floor of the posterior triangle of the neck (splenius 

 capitis). It is covered by the rhomboid muscles, levator scapulae, and serratus 

 posterior superior. 



Nerve-Supply. Posterior rami of cervical and upper thoracic nerves. 



Actions. The splenius cervicis extends the spine, and assists in lateral movement and 

 rotation. The splenius capitis helps in the movements of raising the head, and also of lateral 

 flexion and rotation. 



Second Group. 



M. Sacrospinalis. The sacrospinalis (O.T. erector spinae) possesses vertebral, 

 vertebro-cranial, and vertebro-costal attachments. It consists of an elongated mass 

 composed of separated slips extending from the sacrum to the skull. Simple at its 

 origin, it becomes more and more complex as it is traced upwards towards the head. 



It arises (1) by fleshy fibres from the iliac crest ; (2) from the posterior sacro- 

 iliac ligament ; and (3) by tendinous fibres continuous with the former from the 

 iliac crest, the dorsurn of the sacrum, and the spines of the upper sacral and all 

 the lumbar vertebrae. -Its fibres extend upwards through the loin, enclosed between 

 the posterior and middle layers of the lumbo-dorsal fascia, and separate into two 

 columns a lateral portion derived from the lateral fleshy origin, the iliocostalis, 

 and a medial portion comprising the remaining larger part of the muscle, the 

 longissimus. 



M. Iliocostalis. The iliocostalis lumborum is inserted by six slender slips 

 into the lower six ribs. 



Medial to the insertion of each of these slips is the origin of the iliocostalis 

 dorsi (O.T. accessorius), which, arising from the lower six ribs medial to the 



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