The autochthonous muscles of the back form on each side two longitudinal (lateral and medial) muscular tracts which are lodged in the groove between the vertebral spinous and transverse processes and the angles of the ribs. In the deep parts, nearest to the skeleton, they are made up of short muscles arranged in segments between the vertebrae (medial tract); the long muscles are closer to the surface (lateral tract). In addition the sple-nius muscle covers both tracts in the posterior cervical region. All these mus­cles have a common origin: they originate from the musculature of the back, which in the amphibia consists of a series of myomeres. Beginning with the reptiles, however, only part of the muscles of the back maintain a metameric structure, joining the vertebrae to each other (the muscle of the medial tract), while the other part merges to form the long muscles (the lateral tract). The splenius capitis and splenitis cervicis muscles (m. splenius capitis and m. splenius cervicis) originate from the spinous processes of the lower five cervical and upper six thoracic vertebrae, run upward and laterally, after which the splenius muscle of the head (m. splenius capitis) is inserted into the lateral half of the superior nuchal line and the mastoid process, while the splenius muscle of the neck (m. splenius cervicis) is inserted into the trans­verse processes of the second and third cervical vertebrae. Action. In contraction of one muscle the head turns in its direction; bilateral contraction bends the head and neck backwards. The lateral tract. The characteristic feature of this tract is the attach­ment of muscles to the vertebral transverse processes and the ribs or to their rudiments. 1. The sacrospinalis muscle (m. erector spinae) constitutes the main mass of the autochthonous musculature of the back. It originates from the sacrum, spinous processes of the lumbar vertebrae, iliac crest, and thoracolumbar fascia, and then extends to the skull and forms three divisions according to the insertion: into the ribs, iliocostocervicalis muscle (m. iliocostalis) (the lateral division of the erector spinae). It has three parts: lumbar, inserted into the transverse processes of the upper lumbar vertebrae and the angles of the low­er ribs; thoracic, inserted into the angles of the upper ribs (sixth and fifth); and cervic, inserted into the transverse processes of the lower cervical ver­tebrae; into the transverse processes, the longissimus muscle (m. longissimus) (the intermediate division of the erector spinae). It has four parts (lumbar, thoracic, s. musculi longissimus dorsi, cervicis, and capitis) and is inserted into the transverse processes of all the thoracic and upper cervical vertebrae, the ribs (second to twelfth), and the mastoid process (m. longissimus capitis); into the spinous processes, the spinalis muscle (m. spinalis) (the medial division of the erector spinae). It is inserted into the spinous processes of the thoracic (second to eighth) and cervical (second to fourth) vertebrae. 2. Separate bundles passing between the transverse processes of two adjacent vertebrae are also related to the lateral tract. They are conspicuous in the most mobile parts of the spine: in the cervical (mm. intertransversarii posteriores cervicis) and lumbar (mm. intertransversarii mediales lumbo- rum) parts. The medial tract. The muscles of this tract lie under the lateral tract and consist of separate bundles running obliquely from the transverse processes of one vertebra to the spinous processes of the contiguous vertebra above. Because of this they are known by the common name of transversospinalis muscle (m. transversospinalis). They extend from the sacrum to the occipital bone in three layers that are distinguished by their depth and by the number of vertebrae they overlap. The nearer the muscles to the surface, the steeper and longer the course of their fibres and the greater the number of vertebrae which they overlap. According to this, the following layers are distinguished: a superficial layer (m. semispinalis) whose bundles overlap five or six verte­brae; a middle layer (mm. multifidi) whose bundles overlap three or four ver­tebrae, and a deep layer (mm. rotatores) passing over one vertebra or to the contiguous vertebra. Muscle bundles between spinous processes of contiguous vertebrae, muscles interspinales (mm. interspinales), also belong to the me­dial tract; they are well developed only in the most mobile parts of the spine, i.e. the cervical and lumbar, and are absent in the thoracic region. Their remnant between the sacrum and coccyx is represented by the sacrococcygeus posterior muscle (m. sacrococcygeus posterior). In the most mobile part of the spine, in its articulation with the occipi­tal bone, m. transversospinalis is particularly well developed and is made up of four paired muscles, two oblique and two straight muscles lying to the back of the atlanto-occipital joint under m. semispinalis and m. longissimus. The oblique muscles differ from one another in location and are divided into the superior and inferior muscles. The obliquus capitis superior muscle (m. obliquus capitis superior) stretches from the transverse process of the atlas to the inferior nuchal line. The obliquus capitis inferior muscle (m. obliquus capitis inferior) passes from the spinous process of the second cervical vertebra to the transverse process of the first cervical vertebra (Fig. 126). The straight muscles are distinguished by their length and are divided into the greater and lesser muscles. The greater (m. rectus capitis posterior major) passes from the spinous process of the second cervical vertebra to the inferior nuchal line. The lesser (m. rectus capitis posterior minor) stretches to the same line from the posterior tubercle of the first cervical vertebra. These muscles exert their action on the atlanto-occipital and atlanto-axial joints. Contraction of the muscles on one side contributes to turning of the head to the corre­sponding side. Bilateral contraction pulls the head backward. The action of the complex of the autochthonous muscles of the back consists in holding the trunk erect. The specific features of their insertion by many bundles into many points on the bones ensure the distribution of the muscle force over a large area. Contraction of all the parts of these muscles on both sides effects general extension of the spine, while contraction of separate parts on one side effects extension between the separate vertebrae. Contrac­tion of these muscles on one side causes lateral flexion of the spine and the trunk together with it to this side. The oblique bundles of the autochthonous muscles (mm. rotatores and multifidi) accomplish rotation of the spine. The upper parts of the muscles, nearest to the skull, participate in the correspond­ing places to movements of the head, as pointed out above. The deep muscles of the back also take part in the respiratory excursions. The lower part of the iliocostal muscle lowers the ribs, while its upper part raises them. It should be noted that the erector spinae muscle contracts not only in extension of the spine, but also in flexion of the trunk to counteract the backward pull of gravitational force. Innervation: posterior branches of the spinal nerves, the cervical, thoracic, and lumbar nerves, respectively.