414 THE MUSCULATURE 



region it is somewhat less compact, but is thicker and contains more fat. It is usually divisible 

 into two layers, of which the deeper is adherent to the lumbodorsal fascia. 



The splenius (fig. 380) is enveloped in a thin, adherent fascial covering. The sacro-spinalis 

 is covered by a fascia, the fascia lumbo-dorsalis (fig. 380), which inferiorly is attached to the 

 iliac crest, the distal and lateral margins of the sacrum, and the sacral spines. In the lumbar 

 and thoracic regions it is attached medially to the vertebral spines. Laterally, in the lumbar 

 region, it is reflected around the muscle to its ventral surface, where a 'ventral' or 'deep' 

 layer forms an intermuscular septum (fig. 384) between the quadratus lumborum and the sacro- 

 spinalis. This intermuscular septum (fig. 383) extends from the twelfth rib to the iliac crest 

 and the ilio-lumbar ligament, and is attached medially to the transverse processes of the lumbar 

 vertebrae, from which fibre-bands extend laterally into it. It is strengthened above by fibre- 

 bundles which pass from the first and second lumbar vertebrae to the twelfth rib (lumbo-costal 

 ligament). (For the relation of the abdominal muscles to this fascia see p. 328.) 



In the thoracic region (fig. 384) the lumbo-dorsal fascia is attached to the ribs lateral to 

 the ilio-costal muscle. In the cervical region (fig. 351) the fascia is continued into the inter- 

 muscular septa which surround the muscles of this group in the neck. 



The transversospinal muscles are covered throughout their extent by a fascial membrane 

 which serves to separate them from the longissimus in the sacral, lumbar, and thoracic regions. 



In the dorsal region of the neck (figs. 347, 351, 357) the muscles are covered on each surface 

 by adherent fascial sheets, fascia nuchae, and are arranged in several concentric layers, each 

 of which is separated from its neighbours by dense fatty areolar tissue. The deepest of the 

 layers is formed by the muscles of the transverso-spinal group. This is covered by a dense mem- 

 brane, and is separated from the semispinalis capitis (complexus) by a thick layer of areolar 

 tissue containing the chief blood-vessels and nerves of the neck. The semispinalis capitis 

 (complexus) is covered on each surface by a more delicate adherent membrane, and is separated 

 from the splenius by loose tissue. The splenius has a somewhat denser adherent fascial cover- 

 ing into which the fascia of the levator scapulae is continued. Separated from this by areolar 

 tissue Ues the trapezius. The cervical and thoracic portions of the semispinalis are separated 

 by delicate membranous septa from the semispinalis capitis (complexus), the levator scapulae, 

 and the splenius. The muscles of each side are separated in the dorsal median plane by the 

 dense ligamentum nuchae, into which the various cervical septa and fascia? extend. The sub- 

 occipital muscles are covered by fascial sheaths which are so fused as to constitute a special 

 fascia for these muscles. Distally this is continued into the fascia of the transversospinal 

 muscles. 



MUSCLES 



A. Superficial Lateral Dorsal System 



The splenius (fig. 380). — The two parts of which this muscle is composed may be separately 

 considered. 



The splenius cervicis. — Origin. — By a narrow aponeurotic band from the spinous processes 

 and the supraspinous ligament of the third to the sixth thoracic vertebrae. 



Structure and insertion. — The fibre-bundles extend upward and laterally and give rise to a 

 flat muscle sheet from which fascicuH arise that are inserted by short tendinous processes on the 

 posterior tubercles of the transverse processes of the fu-st two or three cervical vertebrae. The 

 processes are often united with those of the levator scapulae and the longissirnus cervicis. 



The splenius capitis. — Origin. — From the ligamentum nuchae in the region of the third 

 to the seventh cervical vertebrae and from the spinous processes and the supraspinous ligament 

 of the first two to five thoracic vertebrae. 



Structure and insertion. — The fibre-bundles form a sheet which continues cranialward 

 that of the splenius cervicis. The fibre-bundles converge somewhat and are inserted by a 

 short, broad, thick tendon into — (1) the back, the side, and the tip of the mastoid process below 

 the sterno-cleido-mastoid muscle, and (2) into the neighbouring part of the occipital bone. 



Relations. — The splenius lies dorsal to the semispinalis capitis (complexus) and to the 

 cervical (transversalis cervicis) and the cranial (trachelo-mastoid) portions of the longissimus 

 and the cervical portion (cervicalis ascendens) of the ilio-costalis and to the levator scapulae, and 

 is partly covered by the trapezius, sterno-cleido-mastoid, serratus posterior superior, and the 

 rhomboids. In the triangle bounded by the trapezius, sterno-cleido-mastoid, and the levator 

 scapulae it is subcutaneous 



Nerves upply. — 'J'he lateral branches of the posterior divisions of the second, third and fourth 

 (sometimes alsoOf the first, the fifth and the sixth) cervical nerves give off rami which enter the 

 deep surface of the muscle. 



Action.— To incline and rotate the head and neck toward the side on which the muscle is 

 placed. When both muscles act, the head and neck arc extended. 



Variations. — The extent of separation and of fusion of the two muscles varies. Absence 

 of either muscle is rare. The splenius capitis may be divided into mastoid and occipital portions. 

 The attachment of the muscle also varies somewhat. Occasionally the spinal origin of the 

 splenius may extend to the cranial end of the ligamentum nu(;ha!. The origin may extend later- 

 ally over the fascia covering the deeper dorsal muscles. An accessory slip, the splenius cervicis 

 accessorius, separated from the main muscle l)y the tendon of tlie serratus [)osterior superior, 

 is frecjuently (8 per cent, of instances, LeDoul)le) found to run from the lower cervical or upper 

 thoracic vertebrae to the iran.svcrse process of the atlas. 



The sacro-spinalis (erector spinas). — Origin. — (1) From a strong aponeurosis attached to 

 the spines of the lumbar, and the sacral vcrtcbrie, to the ligament passing from the sacrum to the 

 coccyx, to the lateral sacral crest, the sacro-tuberous ligament, the long posterior sac^ro-iliac 

 ligament, and to the dorsal fifth of the iliac crest; (2) directly from the iliac crest in front of 

 and lateral to the attachment of the aponeurosis; and (3) from the short posterior sacro-iHac 



