354 MUSCLES AND FASCIAE. 



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

 tebral aponeurosis. Then detach the Splenius by separating its attachment to the spinous pro- 

 cesses, and reflecting it outwards. 



The Erector Spinae (Fig. 228), 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 Spinae is narrow and pointed, and its origin 

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

 larged, 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, and are gradually lost in the cervical region, where a 

 number of special muscles are superadded, which are continued upwards to the 

 head, and support it upon the spine. 



The Erector Spinae 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 vertebra, and 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 

 posterior part of the sacrum, which represent 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 vertebrae, and by the middle lamella of the aponeurosis of origin of the 

 Transversalis muscle. Opposite the last rib, it divides into two parts, the Sacro- 

 lumbalis, and the Longissimus Dorsi. 



The Sacro-lumbolis (llio-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 upward to the upper ribs, and 

 the cervical portion of the spine. The accessory portions form two additional 

 muscles, the Musculus Accessorius and the Cervicalis Ascendens. 



The Musculus Accessorius ad Sacro-lumbalem arises by separate flattened ten- 

 dons from the angles of the six lower ribs ; these become muscular, and are 

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



The Cervicalis Ascendens 1 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 Loncjissimus Dorsi, the inner and larger portion of the Erector Spinse, 

 arises, with the Sacro-lumbalis, from the common origin above described. In 

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

 the fibres are attached to the whole length of the posterior surface of the trans- 

 verse processes of the lumbar vertebras, to the tubercles at the^ back of the 

 articular processes, and to the layer of lumbar fascia connected with the apices 

 of the transverse processes. In the dorsal region, the Longissimus Dorsi is 

 inserted, by long thin tendons, into the tips of the transverse processes of all 

 the dorsal vertebrae, and into from seven to eleven ribs between their tubercles 

 and angles. This muscle is continued upwards, to the cranium and cervical por- 



1 This muscle is sometimes called " Cervicalis Dcscendens." The student should remember 

 that these long muscles take their fixed point from above or from below, according to circum- 

 stances. 



