348 THE MUSCLES AND FASCIA 



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

 tion, with the Superior oblique ; by its deep surface, with part of the Rectus capitis 

 posticus minor, the posterior arch of the atlas, the posterior occipito-atlantal liga- 

 ment, and part of the occipital bone. 



The Rectus capitis posticus minor, the smallest of the four muscles in this 

 region, is of a triangular shape ; it arises by a narrow pointed tendon from the 

 tubercle on the posterior arch of the atlas, and, becoming broader as it ascends, is 

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

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



Relations. By its superficial surface, with the Complexus and the Rectus 

 capitis posticus major; by its deep surface, with the posterior occipito-atlantal 

 ligament. 



The Oblictuus 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 up- 

 ward, to be inserted into the lower and back part of the transverse process of the atlas. 



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

 terior division of the second cervical nerve, which crosses it ; by its deep surface, 

 with the vertebral artery and posterior atlanto-axial ligament. 



The Obliquus capitis superior, narrow below, wide and expanded above, arises 

 by tendinous fibres from the upper surface of the transverse process of the atlas, 

 joining 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. 



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

 and occipital artery. By its deep surface,v;\i\\ the posterior occipito-atlantal ligament. 



The Suboccipital Triangle. Between the two oblique muscles arid the Rectus 

 capitis posticus major a triangular interval exists, the suboccipital triangle. This 

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

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

 the Obliquus capitis inferior. It is covered in by a layer of dense fibro-fatty tissue, 

 situated beneath the Complexus muscle. The floor is formed by the posterior oc- 

 cipito-atlantal ligament and the posterior arch of the atlas. It contains the verte- 

 bral artery, as it runs in a deep groove on the upper surface of the pos- 

 terior arch of the atlas, and the posterior division of the suboccipital nerve. 



Nerves. The third, 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 dorsal 

 region of the spine ; when only one muscle contracts, it helps to bend the dorsal 

 portion of the spine to one side. The Erector spinae, comprising the Ilio-costalis 

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

 to maintain the spine in the erect posture ; it also serves to bend the trunk back- 

 ward 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 spine being drawn 

 backward by the counterbalancing action of the Erector spinse 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 Ilio-costalis and 

 Longissimus dorsi of one side act, they serve to draw down the chest and spine 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 Transversalis cervicis, when both muscles act, 

 taking their fixed point from below, bend the neck backward. The Trachelo- 

 mastoid, 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 



