416 THE MUSCULATURE 



In the lower lumbar region the sacro-spinalis (erector spinae) muscle begins to show a 

 distinct division into its two chief component parts, the ilio-costalis and the longissimus. 

 The parts of which the ilio-costalis and longissimus are composed will be taken up separately. 

 The ilio-costalis lumborum (figs. 381, 382). — Origin. — (1) Chiefly from the back of the 

 sacrospinal aponeurosis, medial to and cranialward from the iliac crest, and (2) from the iliac 

 crest directly. The deep medial surface of the muscle is closely united in the lumbar region 

 to the longissimus. 



Structure and insertion. — From the mass of fibre-bundles which compose the muscle, fas- 

 ciculi are given off which are attached chiefly by tendinous slips to — (l)the tips of the transverce 

 processes of the lumbar vertebrse; (2) the fibrous processes which extend lateralward from the 

 tips of the transverse processes of the upper lumbar vertebrae into the anterior layer of the lumbo- 

 dorsal fascia; (3) the inferior margin of the last six or seven ribs near the angles. The inser- 

 tions into the lumbo-dorsal fascia and the twelfth rib are usually fleshy. The portions attached 

 to the lumbar vertebrae are by some considered to belong to the longissimus (Eisler). 



Relations. — The muscle lies on the lateral margin of the longissimus and upon the ribs and 

 the external intercostal and levatores costarum muscles, and under the axio-appendicular 

 muscles described above. 



The ilio-costalis dorsi (accessorius). — Origin. — By fleshy fascicuU from the superior borders 

 of the lower seven ribs medial to the angles. 



Structure and insertion. — The slips of origin lie beneath the preceding portion of the muscle, 

 pass medial to and partly fuse with it, and give rise to a belly from which tendinous slips extend 

 to be inserted into the upper seven ribs near their angles and to the transverse process of the 

 seventh cervical vertebra. 



Relations.— The muscle lies upon the ribs and the external intercostal muscles lateral to the 

 longissimus. 



The ilio-costalis cervicis (cervicalis ascendens). — Origin. — By fleshy slips from the upper 

 borders near the angles of the seventh to the third (sometimes to the second or first) ribs. 



Structure and insertion. — The slips of origin are covered by the slips of insertion of the dorsal 



portion (accessorius). They emerge medial to them and give rise to a fleshy belly from which 



tendons pass to the backs of the transverse processes of the sixth to the fourth cervical vertebrae. 



Relations. — The scalenus posterior lies in front, the levator scapulae at the side, and the 



splenius and longissimus (transversalis) cervicis medial to this muscle. 



A bursa is frequently found between the muscle and the tubercle of the first rib. 

 The longissimus dorsi (figs. 381, 382). — Origin. — (1) From the deep surface of the sacro- 

 spinal aponeurosis; (2) from the short posterior sacro-iliac ligaments; and (3) through accessory 

 slips which arise from the transverse processes of the first two lumlaar and the last five or six 

 thoracic vertebrae. In the lumbar region it is fused dorso-laterally with the ilio-costalis. 



Structure and insertion. ^From. the muscle mass arise fasciculi which are inserted partly 

 directly, partly by means of tendons, into — -(1) the lower border of the back of the transverse 

 processes of the lumbar vertebrae and the inferior margins of the ribs lateral to the tubercles; and 

 (2) the accessory tubercles of the lumbar and the tips and inferior margins of the transverse 

 processes of the thoracic vertebrae. The attachment to the first rib is usually wanting. The 

 attachment to the first five ribs may fail. The medial attachments seldom extend to the first 

 vertebra. 



Relations. — The lateral margin of the muscle is covered by the ilio-costalis. Medially it 

 overlies the transverso-spinal muscles. The lateral branches of the dorsal veins, arteries, and 

 nerves pass mainly in the fibrous tissue which separates the longissimus from the ilio-costalis, 

 the medial branches chiefly between the longissimus and the transverso-spinal muscles. The 

 relations to the axio-appendicular muscles and to the dorsal fascia have been pointed out above. 

 Ventrally it lies upon the intertransverse muscles, the external intercostals, and the levatores 

 costarum. 



The longissimus cervicis (transversalis cervicis). — Origin. — By tendinous slips from the 

 transver.sc processes of the first four to six thoracic vertebrae. 



Structure and insertion. — The fasciculi which arise from these slips give rise to a muscle belly 

 from which tendons of insertion extend to the posterior tubercles of the transverse processes of 

 the mid-cervical (second to sixth) vertebrae. 



Relations. — This muscle lies between the longissimus dorsi and capitis with which it is to 

 some extent fused and the ilio-costalis dorsi (accessorius) and cervicis (cervicalis ascendens) 

 muscles. 



The longissimus capitis (trachelo-mastoid). — Origin. — By tendinous slips from the trans- 

 verse processes of the first three or four thoracic vertebrae and the articular processes of the last 

 four cervical. 



Structure and insertion. — The muscle fasciculi arising from these tendons form a belly which 

 is united to the mastoid process by a short tendon. A tendinous inscription often crosses the 

 muscle. 



Relations. — It lies ventral to the splenius capitis, lateral to the semispinalis capitis (com- 

 plexus) and mcrlial to the longissimus cervicis (cervicalis ascendens). 



Nerve-supply of the sacro-spinalis. — From the lateral branches of the posterior divisions of 

 the spinal nerves. The exact distribution of th(«e branches is too complex to be treated here. 

 The nerves for the ilio-costalis arise from the eighth cervical to the first lumbar, those for the 

 longissimus from the first cervical to the fiftli lumbar. 



Action of the sacro-s/>iii,ali.s.— The sacro-si)iiialis serves, when ac^ting on one side, to bend 

 the spinal column toward that side, and when a(;tiiig on botli sides, to extend the spinal column. 

 The cranial portions of the muscle .serve to incline the head toward the same side, and when 

 both muscles act they serve to extend the lu^ad. The ilio-costalis iiuiscle has the greatest power 

 for producing lateral inclination. The ilio-costalis lumborum depresses the ribs, while the 

 ilio-costalis cervicis (cervicalis ascendens) may aid in elevating them. The spinalis muscle 

 serves merely as an extensor. 



