412 THE MUSCULATURE 



The superficial lateral dorsal musculature consists of the splenius and the sacro- 

 spinaHs. The splenius (fig. 380) is a flat, somewhat triangular muscle, which 

 extends from the cervical and upper thoracic spines to the upper cervical trans- 

 verse processes and to the mastoid process of the temporal bone and the neighbour- 

 ing part of the occipital. The sacro -spinalis (erector spinie) (fig. 381) is the name 

 given to a mass of musculature which takes its origin from the ilium, the sacrum, 

 and the lumbar spines. In the lumbar region this muscle divides into its two chief 

 portions, the iUo-costalis and the longissimus. The ilio-costalis (fig. 382) is 

 attached to the lumbar transverse processes and to the ribs near the angles, and is 

 continued upward by accessory fasciculi along the back of the thorax to the trans- 

 verse processes of the cervical vertebrae. The longissimus (fig. 382) extends up- 

 ward between the ilio-costalis and the spines of the lumbar and thoracic vertebrae. 

 It is attached to the transverse processes of the lumbar and thoracic vertebrae and 

 to the ribs lateral to the transverse processes. It is continued to the transverse 

 processes of the cervical vertebrae and to the skull by accessory muscle slips. 



The deep lateral dorsal musculature consists of the dorsal intertransverse 

 muscles. The intertransverse muscles are best developed in the cervical and lum- 

 bar regions. In the cervical region intertransverse muscles belonging to the dor- 

 sal musculature extend between the successive dorsal tubercles, while intertrans- 

 verse muscles belonging to the ventral musculature extend between the ventral 

 tubercles. The latter, as well as the rectus capitis anterior and rectus capitis 

 lateralis, which belong in the series, have been described above (p. 356). In the 

 lumbar region there are also two sets of intertransverse muscles, one belonging to 

 the ventral and one to the dorsal musculature. 



The superficial medial dorsal musculature consists of the spinalis dorsi and 



cervicis. The spinalis dorsi (fig. 381) is intimately fused with the longissimus. It 



extends from the lower to the upper thoracic spines, and is derived from the medial 



dorsal musculature. The inconstant spinalis cervicis, which extends from the 



upper thoracic to the lower cervical spines, is likewise derived from the medial 



dorsal musculature, but is less intimately related to the longissimus. 



. ITie deep medial dorsal musculature (fig. 383) lies in the groove between the 



/ transverse processes and the spines of the sacral, lumbar, thoracic, and cer- 



l vical vertebrae. It extends from the sacrum to the skull, and is best developed in 



Lthe lumbar and cervical regions. It is subdivided into a vertebro-occipital muscle 



(semispinalis capitis), a transverso-spinal group, and the interspinal muscles. 



The semispinalis capitis (complexus) (fig. 381) arises from the transverse processes 



of the third cervical to the sixth thoracic vertebrae and from the spines of the upper 



thoracic vertebrae and is inserted into the base of the skull. The transverso-spinal 



group (fig. 383) extends from the sacrum to the second cervical vertebra. It is 



more or less artificially divisible into several layers. In the superficial layer, the 



semispinalis dorsi et cervicis, which extends from the twelfth thoracic to the second 



cervical vertebra, the constituent fasciculi extend from the transverse process of 



one vertebra to the spine of a vertebra four to six segments above. In the middle 



^ layer, the multifidus, the fasciculi extend over from t\YO to four vertebrae. In the 



deepest layer, the rotatores, the fasciculi extend to the next vertebra (short rotators) 



or to the second vertebra above (long rotators) . The interspinal mustrlesextend 



between successive spines. 



The muscles which pass from the first two vertebrae to the base of the skull 

 behind, or suhoocipiial muscles (fig. 382), consist of the rectus capitis posterior 

 minor, from the spine of the atlas to beneath the inferior nuchal line of the occip- 

 ital and rectus capitis posterior 7najor, from the spine of the epistropheus (axis) to 

 beneath the inferior nuchal line, lateral to the preceding; of the obliquus capitis 

 inferior, from the spine of the epistropheus (axis) to the transverse process of the 

 atlas, and the obliquus capitis superior, from the transverse process of the atlas to 

 the base of the lateral part of the inferior nuchal line of the occipital above the 

 rectus major. 



The primitive condition of the dorsal musculature is one of metameric segmentation. This 

 i.s characteristic of fishes, many amphibia, and of the eml)ryos of all higher vertebrates. In 

 t h(! tailless amphibia, however, a partial differentiation of the dorsal imisculature takes place dur- 

 ing embryonic development, and in all hip;h(!r forma a dilTerentiation takes place which corre- 

 sponds in many ways to that described alcove for man. Acciording to Favaro, the splenius is 

 difTerentiatied from the medial dorsal system, but its innervation should place it with the lateral 

 system. In the human embryo the dorsal segmental musculature extends into the tail region, 

 but afterward here undergoes retrograde metamorphosis. 



