MUSCLES AND FASCIA. 



FOUETH LAYER. 



Sacral and Lumbar Regions. Cervical Region. 



Erector spinae. Cervicalis ascendens. 



Dorsal Region. Transversalis colli. 



Sacro-lumbalis. Trachelo-mastoid. 



Musculus accessorius ad sacro-lumbalem. Complexus. 



Longissimus dorsi. Biventer cervicis. 



Spinalis dorsi. Spinalis cervicis. 



Dissection. To expose the muscles of the fourth layer, the Serrati and vertebral aponeurosis 

 should be entirely removed. The Splenius may then be detached by separating its attachments 

 to the spinous processes, and reflecting it outwards. 



The Erector Spinse (fig. 161), 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 aponeurosis; 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. This large muscular and tendinous 

 mass varies in size and structure at different parts of the spine. In the sacral 

 region, the Erector spinas is narrow and pointed, and its origin chiefly tendinous 

 in structure. In the lumbar region, it 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 vertebra? and ribs, and are 

 gradually lost in the cervical region, where a number of special muscles are 

 superadded, which are continued upwards to the head, which they support upon 

 the spine. 



The Erector spinse arises from the sacro-iliac groove, and 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 three lower 

 dorsal vertebrae, and the supra-spinous 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, representing the transverse processes, where it blends 

 with the great sacro-sciatic ligament. The muscular fibres form a single large 

 fleshy mass, bounded in front by the transverse processes of the lumbar vertebras, 

 and by the middle lamella of the fascia of the Transversalis muscle. Opposite the 

 last rib, it divides into two parts, the Sacro-lumbalis, and the Longissimus dorsi. 

 * The Sacro-lumbalis (Ilio-costalis), the external and smaller portion of the 

 Erector spinae, is inserted, by six or seven flattened tendons, into the angles 

 of the six lower ribs. If this muscle is reflected outwards, 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 Sacro-lumbalis is continued upwards, to be 

 connected with the upper ribs, and with the cervical portion of the spine, 

 forming two additional muscles, the Musculus accessorius and the Cervicalis 

 ascendens. 



The Musculus accessorius ad sacro-lumbalem arises by separate flattened tendons, 

 from the angles of the six lower ribs ; these become muscular, and are finally in- 

 serted, by separate tendons, into the angles of the six upper ribs. 



The Cervicalis ascendens is the continuation of the Accessorius upwards into the 

 neck: it is situated on the inner side of the tendons of the. Accessorius, arising 

 from the angles of the four or five upper ribs, and is inserted, by a series of 

 slender tendons, into the posterior tubercles of the transverse processes of the 

 fourth, fifth, and sixth cervical vertebrae. 



The Longissim,us Dorsi, the inner and larger portion of the Erector spinaa, 

 arises, with the Sacro-lumbalis, from the common origin already mentioned. In 

 the lumbar region, where it is as yet blended with the Sacro-lumbalis, some of 

 the fibres are attached to the posterior surface of the transverse processes of the 

 lumbar vertebrae their whole length, to the tubercles at the back of the articular 



