414 THE MUSCLES 



to a transverse process, e.g. the mastoid process; and the transverso-spi nales. wliich 

 run upwards and inwards from transverse process to spine. 



The action of these muscles will be easily ascertained by observing to which class 

 they belong. The spinales will simply extend, while the transrerscdes will produce 

 lateral flexion as well as extension. The oblique, in addition to extension and 

 lateral flexion, will also produce rotation wherever the ligaments and processes of 

 the vertebrae permit of this movement. 



Fourth Layer 



The fourth layer of the muscles of the back consists of two muscles, Avhich are 

 closely connected with each other, and belong to the spino-transversalis class of the 

 oblique division. 



1 AND 2. SPLENIUS 



The splenius (fig. 305) — named from a-Ay^tov a bandage or compress, probably 

 on account of the way in which it crosses over and l)inds down the complexus and 

 other muscles — is a broad four-sided sheet, divided into two parts at its insertion: 

 the one part being the splenius capitis, and the other the splenius colli. 



(1) Splenius Capitis 



Origin. — (1) The lower two-thirds of the ligamentum nuchae; (2) the spines of 

 the seventh cervical and upper two thoracic vertebra?, and the interspinous liga- 

 ments between them. 



Insertion. — The outer third of the middle nuchal line of the occipital bone, 

 and the back part of the outer surface of the mastoid process of the temjjoral 

 bone. 



Structure. — Arising Ijy a short tendinous aponeurosis, the fleshy fibres pass 

 upAvards and outwards parallel to each other, and are inserted by short tendinous 

 fibres, except along the anterior border, where the tendon of insertion extends more 

 than an inch (2-5 cm.) downwards. 



Nerve-supply. — The external branches of the posterior primary divisions of 

 the middle cervical nerves which enter the outer part of the deep surface of the 

 muscle. 



Action. — (1) To extend the head and neck; (2) to flex them laterally; (3) to 

 rotate the face to the same side. 



(2) Splenius Colli 



Origin. — The spines of the third to the sixth thoracic vertebrae, and the inter- 

 spinous ligaments between them. 



Insertion. — The back of the posterior tul^ercles of the transverse processes of 

 three or four upper cervical vertebrae. 



Structure. — Of parallel fleshy fibres witli a short aponeurotic origin, and 

 inserted liy small tendons, the highest of which is the largest. 



Nerve-supply. — The external branches of the i)osterior divisions of the loAvcr 

 cervical nerves. 



Action. — ( 1) To extend the upper ])art of the neck; (2) to flex it laterally; 

 (3) to rotate it to llic same side. 



Relations of the splenius. — Superficially, the sterno-mastoid, trajiezius. ser- 

 ratus posticus superior, and the levator anguli scapula? which conceals the splenius 

 colli in the posterior triangle of the neck. Beneath lie the complexus, trachelo- 

 mastoid, cervicalis ascendens, and transversalis colli. 



Variations. — The number of the thoracic vertebrre from which the splenii arise varies. The 

 splenius colli may be absent. The slip mentioned in the variations of the serratus posticus 

 superior may be considered to be a variation of the splenii. 



