OF THE BACK. 343 



and Complexus ; acting separately, they draw the head to one or the other side, 

 and slightly rotate it, turning the face to the same side. They also assist in sup- 

 porting the head in the erect position. 



Fourth Layer. 



I. Erector spinae. 



a. Outer Column. b. Middle Column. 



Ilio-costalis. Longissimus dorsi. 



Musculus accessorius. Transversalis cervicis. 



Cervicalis ascendens. Trachelo-mastoid. 



c. Inner Column. 

 Spinalis dorsi. 



II. Complexus. 



Dissection. To expose the muscles of the fourth layer, remove entirely the Serrati and the 

 vertebral and lumbar fasciae. Then detach the Splenius by separating its attachment to the 

 spinous processes and reflecting it outward. 



The Erector spinse (Fig. 214) and its prolongations in the dorsal and cervical 

 regions fill up the vertebral groove on each side of the spine. It is covered in 

 the lumbar region by the lumbar fascia ; in the dorsal region, by the Serrati 

 muscles and the vertebral aponeurosis ; and in the cervical region, by a layer of 

 cervical fascia continued beneath the Trapezius and the Splenius. This large 

 muscular and tendinous mass varies in size and structure at different parts of the 

 spine. In the sacral region the Erector spinae is narrow and pointed, and its origin 

 chiefly tendinous in structure. In the lumbar region the muscle becomes enlarged, 

 and forms a large fleshy mass. In the dorsal region it subdivides into two parts, 

 which gradually diminish in size as they ascend to be inserted into the vertebrae 

 and ribs. 



The Erector spinse arises from the anterior surface of a very broad and thick 

 tendon, which is attached, internally, to the spines of the sacrum, to the spinous 

 processes of the lumbar and the eleventh and twelfth dorsal vertebrae, and. the su- 

 praspinous ligament ; externally, to the back part of the inner lip of the crest of 

 the ilium, and to the series of eminences on the posterior part of the sacrum, which 

 represents the transverse processes, where it blends with the great sacro-sciatic and 

 posterior sacro-iliac ligaments. Some of its fibres are continuous with the fibres 

 of origin of the Gluteus maximus. The muscular fibres form a single large fleshy 

 mass, bounded in front by the transverse processes of the lumbar vertebrae and by 

 the middle lamella of the lumbar fascia. Opposite the last rib it divides into two 

 parts, the Ilio-costalis and the Longissimus dorsi ; the Spinalis dorsi is given off 

 from the latter in the upper dorsal region. 



The Ilio-costalis (Sacro-lumbalis), the external portion of the Erector spinae, is 

 inserted, generally, by six or seven flattened tendons into the inferior borders of 

 the angles of the six or seven lower ribs. The number of the tendons of this muscle 

 is, however, very variable, and therefore the number of ribs into which it is in- 

 serted. Frequently it is found to possess nine or ten tendons, and sometimes as 

 many tendons as there are ribs, and is then inserted into the angles of all the ribs. 

 If this muscle is reflected outward, it will be seen to be reinforced by a series of 

 muscular slips which arise from the angles of the ribs ; by means of these the Ilio- 

 costalis is continued upward to the upper ribs and cervical portion of the spine. 

 The accessory portions form two additional muscles, the Musculus accessorius and 

 the Cervicalis ascendens. 



The Musculus accessorius ad ilio-costalem arises, by separate flattened tendons, 

 from the upper borders of the angles of the six lower ribs : these become muscular, 

 and are finally inserted, by separate tendons, into the upper borders of the angles 



