OF THE BACK 41 1 



The Fourth Laypr (Fig. 313). 



I. Erector spinae. 



a. Outer Column. b. Middle Column 



Iliocostalis. Longissimus dorsi. 



Musculus accessorius. Transversalis cervicis. 



Cervicalis ascendens. Trachelomastoid. 



c. Inner Column. 

 Spinalis dorsi. Spinalis colli. 



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 spinae (m. sacrospinalis] and its prolongations in the thoracic 

 and cervical regions fill up the vertebral groove on each side of the vertebral column. 

 It is covered in the lumbar region by the lumbar fascia; in the thoracic 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 vertebral column. In the sacral region the Erector spinae is narrow and 

 pointed, and its origin is chiefly tendinous in structure. In the lumbar region 

 the muscle becomes enlarged, and forms a large, fleshy mass. In the thoracic 

 region it subdivides into two parts, which gradually diminish in size as they ascend 

 to be inserted into the vertebrae and ribs. 



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

 tendon, the erector spinae aponeurosis, which is attached, internally, to the spines 

 of the sacrum, to the spinous processes of the lumbar and the eleventh and twelfth 

 thoracic vertebrae, and to the supraspinous ligament; externally, to the back part 

 of the inner lip of the crest of the ilium, and to the series of eminences on the pos- 

 terior part of the sacrum, which represents the transverse processes, where it 

 blends with the great sacrosciatic and posterior sacroiliac ligaments. Some of 

 its fibres are continuous with the fibres of origin of the Gluteus maximus. The 

 muscle 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 three parts: (1) The Iliocostalis; (2) the Longis- 

 simus dorsi; and (3) the Spinalis dorsi. 



1. The Iliocostalis (m. iliocostalis lumborum), 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, quite variable, and therefore the number of 

 ribs into which it is inserted varies. Internally this muscle is reinforced by a series 

 of muscle slips which arise from the angles of the ribs; by means of these the 

 Iliocostalis is continued upward to the upper ribs and to the cervical portion of 

 the vertebral column. These accessory portions form two additional muscles, 

 the Musculus accessorius and the Cervicalis ascendens. 



The Musculus accessorius (m. iliocostalis dorsi) 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 of the six upper ribs and into the back of the transverse processes 

 of the seventh cervical vertebra. 



