416 THE MUSCLES AND FASCIA 



curved line, nearly as far as the foramen magnum, and nearer to the middle line 

 than the preceding. 



The Obliquus inferior (m. obliquus capitis inferior), the larger of the two 

 Oblique muscles, arises from the apex of the spinous process of the axis, and 

 passes outward and slightly upward, to be inserted into the lower and back part 

 of the transverse process of the atlas. 



The Obliquus superior (m. obliquus capitis superior), narrow below, wide and 

 expanded above, arises by tendinous fibres from the upper surface of the transverse 

 process of the atlas; it joins with the insertion of the preceding, and, passing 

 obliquely upward and inward, is inserted into the occipital bone, between the two 

 curved lines, external to the Complexus. 



The Suboccipital triangle is the tniangular interval between the two oblique muscles and the 

 Rectus capitis posticus major. This triangle is bounded, above and internally, by the Rectus 

 capitis posticus major; above and externally, by the Obliquus superior; below and externally, 

 by the Obliquus inferior. It is covered in by a layer of dense fibrofatty tissue, situated 

 beneath the Complexus muscle. The floor is formed by the posterior occipito-atlantal ligament 

 and the posterior arch of the atlas. It contains the vertebral artery, as it runs in a deep groove 

 on the upper surface of the posterior arch of the atlas, and the posterior division of the sub- 

 occipital nerve. 



Nerves. The fourth and fifth layers of the muscles of the back are supplied by the posterior 

 primary divisions of the spinal nerves. 



Actions. When both the Spinales dorsi contract, they extend the thoracic region of the 

 vertebral column; when only one muscle contracts, it helps to bend the thoracic portion of 

 the vertebral column to one side. The Erector spinae, comprising the Iliocostalis and the 

 Longissimus dorsi with their accessory muscles, serves, as its name implies, to maintain the 

 vertebral column in the erect posture; it also serves to bend the trunk backward when it is 

 required to counterbalance the influence of any weight at the front of the body, as, for instance, 

 when a heavy weight is suspended from the neck, or when there is any great abdominal dis- 

 tention, as in pregnancy or dropsy; the peculiar gait under such circumstances depends upon 

 the vertebral column being drawn backward by the counterbalancing action of the Erector 

 spinae muscles. The muscles which form the continuation of the Erector spinae upward 

 steady the head and neck, and fix them in the upright position. If the Iliocostalis and Lon- 

 gissimus dorsi of one side act, they serve to draw down the thorax and vertebral column to the 

 corresponding side. The Cervicales ascendens, taking their fixed points from the cervical 

 vertebrae, elevate those ribs to which they are attached; taking their fixed points from the ribs, 

 both muscles help to extend the neck; while one muscle bends the neck to its own side. The 

 Transversales cervicis, when both muscles act, taking their fixed point from below, bend the neck 

 backward. The Trachelomastoid, when both muscles act, taking their fixed point from below, 

 bend the head backward; while if only one muscle acts, the face is turned to the side on which 

 the muscle is acting, and then the head is bent to the shoulder. The two Recti muscles 

 draw the head backward. The Rectus capitis posticus major, owing to its obliquity, rotates 

 the cranium, with the atlas, around the odontoid process, turning the face to the same side. The 

 Multifidus spinae acts successively upon the different parts of the vertebral column; thus, the 

 sacrum furnishes a fixed point from which the fasciculi of this muscle act upon the lumbar 

 region; these then become the fixed points for the fasciculi moving the thoracic region, and 

 so on throughout the entire length of the vertebral column; it is by the successive contraction 

 and relaxation of the separate fasciculi of this and other muscles that the spine preserves the 

 erect posture without the fatigue that would necessarily have been produced had this position 

 been maintained by the action of a single muscle. The Multifidus spinae, besides preserving 

 the erect position of the vertebral column, serves to rotate it, so that the front of the trunk is 

 turned to the side opposite to that from which the muscle acts, this muscle being assisted in its 

 action by the Obliquus externus abdominis. The Complexi draw the head directly backward; 

 if one muscle acts, it draws the head to one side, and rotates it so that the face is turned to the 

 opposite side. The Superior oblique draws the head backward, and, from the obliquity in 

 the direction of its fibres, will slightly rotate the cranium, turning the face to the opposite side. 

 The Inferior oblique rotates the atlas, and with it the cranium, around the odontoid process, 

 turning the face to the same side. The Semispinales, when the muscles of the two sides act 

 together, help to extend the vertebral column; when the muscles of one side only act, they rotate 

 the thoracic and cervical parts of the vertebral column, turning the body to the opposite side. 

 The Supraspinales and Interspinales by approximating the spinous processes help to extend 

 the vertebral column. The Intertransversales approximate the transverse processes, and help 

 to bend the vertebral column to one side. The Rotatores spinae assist the Multifidus spinae to 



