420 THE MUSCLES 



Relations. — Supcrticially, the two serrati postici, the latissimus dorsi, the 

 rhoniboicU'i and splenii; externally, the longissimus dort^i; deei)ly the semisj)inalis 

 dorsi and complexus. 



Variations of the erector spinse and its divisions. — The number of ribs or vertebrse from 

 which the various parts of these divisions arise, or into whicli thej' arc inserted, varies much. 

 The insertions upon tlie lumbar vertchnc are often less numerous than those described. Occa- 

 sionally the loiii.Mssimus dorsi receives accessory fibres from the lower ribs. Sometimes the 

 portion of the lonizissimus dorsi, which arises from the transverse i)roces.ses of the lower thoracic 

 vertebrse, is separate from the rest of the muscle, and is inserted into the transverse processes of 

 some of the upper thoracic vertebrse. 



Sixth Layer 



The sixth layer, or rather group, consists of four strata of oblique muscles 

 belonging to the class of transverso-spinales; the deepest stratum being formed by 

 a series of small muscles which run upwards and inwards from the back of one 

 vertebra to that of the next above it; while the other strata run in the same direc- 

 tion but less obliquely, so as to cross over the backs of several vertebra? before 

 reaching their insertion. 



1. COMPLEXUS 



The complexus, or semispinalis capitis (fig. 306) — named from the complex 

 or complicated arrangement of the muscular bundles which were formerly included 

 under this designation, viz. the complexus, trachelo-mastoid, &c. — is a musculo- 

 tendinous band somewhat constricted about its middle. 



Origin. — (1) The back of the articular processes of the cervical vertebrge from 

 the third to the sixth; (2) the back of the transverse processes of the seventh cer- 

 vical and the six upper thoracic vertebrse; (3) generally also by an inner head from 

 the spine of the seventh cervical vertebra. 



Insertion. — The under surface of the occipital bone between the middle and 

 inferior nuchal lines from the middle line for nearly two inches (5 cm.) outwards. 



Structure. — Arising by tendinous slips, the fleshy fibres pass upwards and 

 slightly inwards, and are inserted either directly into the occiput or by a short 

 aponeurosis which covers the thick part of the muscle near its outer border. On 

 their way, the innermost fibres are intersected by a transverse tendinous band 

 opposite the sixth cervical spine, and this part of the muscle, being thus divided, 

 is often called the biventer cervicis. There is usually also a smaller tendinous 

 intersection across the posterior surface of the muscle at a higher level. 



Nerve-supply. — From the suboccipital and great occipital nerves, and from 

 the internal divisions of the posterior branches of the three or four following 

 cervical nerves which enter the deep surface of the muscle. 



Action. — To extend the head, to flex it laterally, and to rotate the face slightly 

 to the opj)osite side. 



Relations. — It is covered by the trapezius and the two splenii, and it lies upon 

 the muscles of the suboccipital triangle and the semispinalis colli, the vertebral 

 artery and the anastomosis between the arteria princeps cervicis and the arteria 

 profunda cervicis. It is pierced by the great occipital nerve. Internally it lies 

 close to the ligamentum nuchse; and externally, to the trachelo-mastoid and 

 transversalis colli. 



Variations. — The inner head may arise from several of the thoracic sjiines as well as that of 

 the seventh cervical vertebra. Sometimes a second and smaller complexus runs beneath the 

 normal muscle from the upper thoracic vertebrae to the head. 



2. SEMISPINALIS DORSI 



The semispinalis dorsi — named from the fact that the inner half only of the 

 muscle, i.e. its insertion, is attached to the spines of the vertebrse, and from its 

 situation in the dorsal region — is a rather feeble compound muscle, consisting of a 

 series of small muscles with tendinous extremities. 



