402 MYOLOGY 



The Obliquus capitis inferior (Obliquus inferior), the larger of the two Oblique 

 muscles, arises from the apex of the spinous process of the axis, and passes lateral- 

 ward and slightly upward, to be inserted into the lower and back part of the 

 transverse process of the atlas. 



The Obliquus capitis superior (Obliquus superior), narrow below, wide and 

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

 process of the atlas, joining with the insertion of the preceding. It passes upward 

 and medialward, and is inserted into the occipital bone, between the superior and 

 inferior nuchal lines, lateral to the Semispinalis capitis. 



The Suboccipital Triangle. Between the Obliqui and the Rectus capitis posterior major is 

 the suboccipital triangle. It is bounded, above and medially, by the Rectus capitis posterior 

 major; above and laterally, by the Obliquus capitis superior; below and laterally, by the Obliquus 

 capitis inferior. It is covered by a layer of dense fibro-fatty tissue, situated beneath the Semi- 

 spinalis capitis. The floor is formed by the posterior occipito-atlantal membrane, and the posterior 

 arch of the atlas. In the deep groove on the upper surface of the posterior arch of the atlas are 

 the vertebral artery and the first cervical or suboccipital nerve. 



Nerves. The deep muscles of the back and the suboccipital muscles are supplied by the 

 posterior primary divisions of the spinal nerves. 



Actions. The Sacrospinalis and its upward continuations and the Spinales serve to main- 

 tain the vertebral column in the erect posture; they also serve 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 

 distension, 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 Sacrospinales. 

 The muscles which form the continuation of the Sacrospinales on to the head and neck steady 

 those parts and fix them in the upright position. If the Iliocostalis lumborum and Longissimus 

 dorsi of one side act, they serve to draw down the chest and vertebral column to the correspond- 

 ing side. The Iliocostales cervicis, 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. When both Longissimi 

 cervicis act from below, they bend the neck backward. When both Longissimi capitis act from 

 below, they 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 

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

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

 Multifidus acts successively upon the different parts of the column; thus, the sacrum furnishes 

 a fixed point from which the fasciculi of this muscle acts upon the lumbar region; which in turn 

 becomes the fixed point for the fasciculi moving the thoracic region, and so on throughout the 

 entire length of the column. The Multifidus also serves to rotate the column, 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 Obliquus capitis superior draws 

 the head backward and to its own side. The Obliquus inferior rotates the atlas, and with it the 

 skull, around the odontoid process, turning the face to the same side. When the Semispinales of 

 the two sides act together, they help to extend the vertebral column; when the muscles of only 

 one side act, they rotate the thoracic and cervical parts of the column, turning the body to the 

 opposite side. The Semispinales capitis 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 

 Interspinales by approximating the spinous processes help to extend the column. The Inter- 

 transversarii approximate the transverse processes, and help to bend the column to one side. 

 The Rotatores assist the Multifidus to rotate the vertebral column, so that the front of the trunk 

 is turned to the side opposite to that from which the muscles act. 



III. THE MUSCLES OF THE THORAX. 



The muscles belonging to this group are the 



Intercostales externi. Levatores costarum. 



Intercostales interni. Serratus posterior superior. 



Subcostales. Serratus posterior inferior. 



Transversus thoracis. Diaphragm. 



Intercostal Fasciae. In each intercostal space thin but firm layers of fascia 

 cover the outer surface of the Intercostalis externus and the inner surface of the 





