358 MUSCLES AND FASCIJB. 



apex of the spinous process of the axis, and passes almost horizontally outwards, 

 to be inserted into the apex of the transverse process of the atlas. 



Relations. By its superficial surface, with the Complexus, and with the pos- 

 terior branch of the second cervical nerve which crosses it. By its deep surface, 

 with the vertebral artery, and posterior atlo-axoid ligament. 



The Obliquus Superior, narrow below, wide and expanded above, arises by 

 tendinous fibres from the upper part of the transverse process of the atlas, 

 joining with the insertion of the preceding, and, passing obliquely upwards and 

 inwards, is inserted into the occipital bone, between the two curved lines, 

 external to the Complexus. Between the two Oblique muscles and the Kectus' 

 Posticus Major a triangular interval exists, in which is seen the vertebral artery, 

 and the posterior branch of the suboccipital nerve. 



Relations. By its superficial surface, with the Complexus and Trachelo- 

 mastoid. By its deep surface, with the posterior occipito-atloid ligament. 



Nerves. The Semispinalis Dorsi and Eotatores Spinae are supplied by the 

 internal posterior branches of the dorsal nerves; the Semispinalis Colli, Supra- 

 spinales, and Interspinales, by the internal posterior branches of the cervical 

 nerves; the Intertransversales, by the internal posterior branches of the cervi- 

 cal, dorsal, and lumbar nerves ; the Multifidus Spinge, by the same, with the 

 addition of the internal posterior branches of the sacral nerves. The Eecti and 

 Obliqui muscles are all supplied by the suboccipital and great occipital nerves. 



Actions. The Erector Spinae, comprising the Sacro-lumbalis, with its acces- 

 sory muscles, the Longissimus Dorsi and Spinalis Dorsi, serves, as its name 

 implies, to maintain the spine in the erect posture ; it also serves to bend the 

 trunk backwards 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 sus- 

 pended from the neck, or when there is any great abdominal development, as 

 in pregnancy or dropsy; the peculiar gait under such circumstances depends 

 upon the spine being drawn backwards, by the counterbalancing action of the 

 Erector Spinae muscles. The muscles which form the continuation of the 

 Erector Spina3 upwards steady the head and neck, and fix them in the upright 

 position. If the Sacro-lumbalis and Longissimus Dorsi of one side act, they 

 serve to draw down the chest and spine to the corresponding side. The Cervi- 

 calis Ascendens, taking its fixed point from the cervical vertebrae, elevates 

 those ribs to which it is attached. The Multifidus Spinaa acts successively 

 upon the different parts of the spine; 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 dorsal region, and so on 

 throughout the entire length of the spine ; 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 

 spine, 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, the analogues 

 of the Multifidus Spinae in the neck, 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 Eectus Capitis Posticus Minor and the Supe- 

 rior Oblique draw the head backwards ; and the latter, from the obliquity in 

 the direction of its fibres, may turn the face to the opposite side. The Eectus 

 Capitis Posticus Major and the Obliquus Inferior rotate the atlas, and, with it, 

 the cranium round the odontoid process, and turn the face to the same side. 



