FOURTH LAYER. 201 



The splenius colli is inserted into the posterior tubercles of the 

 transverse processes of the three or four upper cervical vertebrae. 



Relations. By its superficial surface with the trapezius, sterno- 

 mastoid, levator anguli scapulae, rhomboideus minor and major, and 

 serratus posticus superior. By its deep surface with the spinalis dorsi, 

 longissimus dorsi, semi-spinalis colli, complexus, trachelo-mastoid, and 

 transversalis colli. The tendons of insertion of the splenius colli are 

 interposed between the insertions of the levator anguli scapulae in 

 front, and the transversalis colli behind. 



The splenii of opposite sides of the neck leave between them a trian- 

 gular interval, in which the complexus is seen. 



Fourth Layer. 



Dissection. The two serrati and two splenii muscles must be re- 

 moved by cutting them away from their origins and insertions, to 

 bring the fourth layer into view. 



Three of these muscles, viz. sacro-lumbalis, longissimus dorsi, and 

 spinalis dorsi, are associated under the name of erector spinae. They 

 occupy the lumbar and dorsal portion of the back. The remaining 

 four are situated in the cervical region. 



The SACRO-LUMBALIS and LONGISSIMUS DORSI arise by a common 

 origin from the posterior third of the crest of the ilium, from the pos- 

 terior surface of the sacrum, and from the lumbar vertebrae : opposite 

 the last rib a line of separation begins to be perceptible between the 

 two muscles. The sacro-lumbalis is inserted by separate tendons into 

 the angles of the six lower ribs. On turning the muscle a little out- 

 wards, a number of tendinous slips will be seen taking their origin 

 from the ribs, and terminating in a muscular fasciculus, by which the 

 sacro-lumbalis is prolonged to the upper part of the thorax. This is 

 the muscidus accessorius ad sacro-lumbalem : it arises from the angles of 

 the six lower ribs, and is inserted by separate tendons into the angles 

 of the six upper ribs. 



The longissimus dorsi is inserted into all the ribs, between their 

 tubercles and angles. 



The SPINALIS DORSI arises from the spinous processes of the two 

 upper lumbar and two lower dorsal vertebrae, and is inserted into the 

 spinous processes of all the upper dorsal vertebrae ; the two muscles 

 form an ellipse, which appears to enclose the spinous processes of all 

 the dorsal vertebrae. 



Relations. The erector spinae muscle is in relation by its super- 

 ficial surface (in the lumbar region) with the conjoined aponeurosis of 

 the transversalis and internal oblique muscle, which separates it from 

 the aponeurosis of the serratus posticus inferior, and longissimus 

 dorsi ; (in the dorsal region) with the vertebral aponeurosis, which 

 separates it from the latissimus dorsi, trapezius, and serratus posticus 

 superior, and with the splenius. By its deep surface (in the lumbar 



