ERECTORES SriNiE. COMPLEX US. 301 



continuation of the longissimus dorsi to the head, arises in close con- 

 nection with tlie transversahs cervicis from the upper dorsal transverse 

 processes, and also from behind the articular processes of the three or 

 four lowest cervical vertebra), and, forminc^ a thin flat muscle, passes to 

 be inserted into the posterior margin of the mastoid process, under 

 cover of the splenius and sterno-mastoid muscles. It is the only muscle 

 which lies between the splenins and complexus, and the only portion of 

 the erector spintc concealed by the fonner. 



fl. The spinalis dorsi is a long narrow muscle placed at the inner 

 side of the longissimus dorsi, and closely connected with it. It forms 

 the innermost column of the erector spina?. It arises by several 

 tendons from the spines of the first two lumbar, and the two lowest 

 dorsal vertebrse, and is inserted into from four to eight of the higlier 

 dorsal spines. It adheres closely to the semispinalis muscle upon which 

 it lies. 



Actions; — The splenii and traclielo-mastoid muscles of botli sides acting- 

 together draw backwards the head and upper cervical vertebraj : when the 

 muscles of one side alone act, the extension is accompanied by lateral flexion 

 and rotation, especially of the head on the axis. 



"WTien the greater part of the erector muscles of the spine are in action on botli 

 sides, they bend backwards the vertebral column and trunk ; and these muscles 

 co-operate powerfully in ahnost every great muscidar effort of the body or limbs. 

 Their power to straighten the back from the bent condition, as measui'ed by the 

 muscular dj-namometer, varies in adults of medium strength from 200 lbs. to- 

 400 lbs. By the action of the erector muscles of one side a certain amount of 

 lateral flexion and of rotation, which is greatest in the chest, accompanies felie 

 extension. A certain amount of extension or erection of the spine, as previously 

 stated, accompanies inspkation ; but if the spine be fixed, some of the erector 

 muscles may also, by their costal attachments, depress the ribs, and thus assist in 

 expiration. 



Complexus and tkansverso-spinales. — The muscles of the com- 

 plicated group comprising the complexus, semispinalis, multifidus and 

 rotatores spinte, present the feature in common of ascending with an 

 inward inclination, and are thus distinguished from those last described. 

 The most superficial, the complexus, not usually included in this gi'oup, 

 but obviously resembling the others, has the longest and most vertical 

 fibres, but is the shortest in its whole extent, being limited to the upper 

 dorsal and the cervical region ; the muscle beneath it, the semispinalis, 

 occupies the greater part of both these regions : while the multifidus 

 spina), extending from the sacrum to the axis, has the shortest and most 

 oblique fibres. 



The complexus muscle arises by tendinous points from the trans- 

 verse processes of the seven highest dorsal and the lowest cervical 

 vertebra, and from the articular processes of four and sometimes five 

 other cervical vertebra), together with the capsular ligament uniting 

 them ; and it is inserted into the large internal impression between the 

 two curved lines of the occipital bone. It is narrower above than 

 below, and its inner margin in the neck is in contact with the liga- 

 mentum nuchje. Above its middle the muscle is partly intersected by 

 a tendon. This portion of the muscle is often described separately, 

 under the name of hiventer cervicis : its fibres are also frequently con- 

 nected by a tendinous slip with one or two of the spinous processes 

 belonging to the last cervical or first dorsal vertebrce. 



