ERECTOR SPIX.E—ILIO-COSTALIS 415 



FiFrii Layer 



The fifth layer consists of vertical muscles, and contains both spinales and 

 transversalns. To the hitter ckiss Ijelong the greater part of the erector spina^ and 

 the seven muscles which either spring from it or are serially continuous with it: 

 viz. the ilio-costalis, accessorius ad ilio-costalem, cervicalis ascendens, longissimus 

 dorsi, transversalis colli, trachelo-mastoid, and the spinalis dorsi. 



1. ERECTOR SPIN.E 



The erector spinae — named from its function — is a broad and very strong 

 membranous sheet of a triangular shape, with a layer of fleshy fibres upon its 

 deep surface, and it divides in the lumbar region into three longitudinal series of 

 muscles which fill the greater part of the vertel:)ral groove. 



Origin. — (1) The spines of the two last thoracic, all the lumbar, and the four 

 upper sacral vertebrae; (2) the back of the side portion of the fourth sacral ver- 

 teljra; (3) the posterior sacro-iliac ligament, a few of these fibres being continuous 

 with the origin of the gluteus maximus; (4)' the upper part of the posterior supe- 

 rior spine of the ilium, and the posterior fifth of the iliac crest. 



Insertion. — It is continuous with the spinalis dorsi. longissimus dorsi, and 

 ilio-CDstalis. 



Structure. — A strong membranous sheet, everywhere continuous except at the 

 origin from the lower thoracic and upper lumbar spines, where it is partly divided 

 into separate tendons. 



It breaks up into fleshy fibres in a line which stretches obliquely upwards and 

 inwards from the front of the iliac origin to the last thoracic spines. Fleshy fibres 

 also arise upon its deeper surface at a lower level, and some arise directly from 

 the posterior superior iliac spine and the crest under cover of the tendinous 

 origin. 



Nerve-supply. — The external l)ranches of the posterior divisions of the lumbar 

 nerves which enter its deep surface. 



Action. — Generally, that of extension of the lumbar spine on the pelvis; but a 

 full account will be given with the description of the succeeding muscles. 



Relations. — Superficially, the vertebral aponeurosis, Avith which it is blended 

 at its origin and for the first two inches (5 cm.) of its course. By this aponeurosis 

 it therefore enters into a close relation with the lower part of the tendons of the 

 latissimus dorsi and serratus posticus inferior. Deeply, the nmltifidus spinae, 

 some of the fibres of which arise from it. 



The outer division extends upwards along the angles of the ribs and the trans- 

 verse processes of the lower cervical vertebrae, and consists of the ilio-costalis. 

 accessorius ad ilio-costalem, and the cervicalis ascendens. It is separated from 

 the middle division by the external branches of the posterior divisions of the sj^innl 

 nerves. 



Outer Division 



2. ILIO-COSTALIS 



The ilio-costalis, or sacro-lumbalis — named from its attachment to the ilium 

 and ribs — is an elongated muscular sheet, with a serrated outer border formed Ijy 

 the insertion of its tendons into the lower rilxs. 



Origin. — The outer portion of the erector spinse. 



Insertion. — (1) The angles of all the ribs from the sixth to the eleventh; 

 (2) the lower border of the last rib; (3) the tips of the transverse processes of 

 the lumbar vertebrae, and the fibrous processes which extend outwards from 

 the tips of the transverse processes of the upper lumbar vertebrae into the 

 lumbar fascia. 



