280 MUSCLES AND FASCIAE. 



into the rough surface beneath the inferior curved line, nearly as far as the fora- 

 men magnum, nearer to the middle line than the preceding. 



Relations. By its superficial surface, with the Complexus. By its deep surface, 

 with the posterior occipito-atloid ligament. 



The Obliquus Inferior, the largest of the two oblique muscles, arises from the 

 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 is crossed by the 

 posterior branch of the second cervical nerve. By its deep surface, with the ver- 

 tebral artery, and posterior occipito-atloid ligament. 



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

 dinous 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 Eectus posticus major, a 

 triangular interval exists, in which are 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 Semi-spinalis dorsi and Eotatores spinae are supplied by the 

 internal posterior branches of the dorsal nerves. The Semi-spinalis colli, Supra- 

 spinales, and Inter-spinales, by the internal posterior branches of the cervical 

 nerves. The Inter-transversales, by the internal posterior branches of the cervical, 

 dorsal, and lumbar nerves. And the Multifidus spinae, 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 accessory 

 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 suspended from the neck, or 

 when there is any great abdominal development, as in pregnant women or in 

 abdominal dropsy ; the peculiar gait under such circumstances depending upon 

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

 spinal muscles. The continuation of these muscles upwards to the neck and head 

 steady and preserve the upright position of these several parts. 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 Musculus accessorius, taking its 

 fixed point from the cervical vertebrae, elevates those ribs to which it is attached. 

 The Multifidus spinae acts successively upon the different segments of the spine ; 

 thus, the lateral parts of the sacrum furnish 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 with- 

 out the fatigue that would necessarily have existed had this movement been 

 accomplished 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 backwards ; 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 Superior 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. 





