440 THE MUSCLES AND FASCIA. 



Supraspinalesandlnterspinales are supplied by the internal branches of the posterior 

 divisions of the cervical, dorsal, and lumbar nerves in the respective regions ; the 

 Intertransversales, by the internal branches of the posterior divisions of the cervical, 

 dorsal, and lumbar nerves ; the Multifidus spinge, by the same, with the addition 

 of the internal branches of the posterior divisions of the sacral nerves. The Recti 

 and Obliqui muscles are all supplied by the suboccipital nerve ; the Inferior oblique 

 is also supplied by the great occipital nerve. 



Actions. When both the Spinales dorsi contract, they extend the dorsal 

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

 portion of the spine to one side. The Erector spinge, 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 spinge muscles. The 

 muscles which form the continuation of the Erector spinge 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 vertebras, 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 colli,. 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 

 muscles draw the head backward. The Rectus capitis posticus major, owing to 

 its obliquity, rotates the cranium, with the atlas, round the odontoid process, 

 turning the face to the same side. The Multifidus spinge 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 spinge, 

 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 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 Obliquus capitis inferior rotates the atlas, and with it the 

 cranium, round 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 

 spine ; when the muscles of one side only act, they rotate the dorsal and cervical 

 parts of the spine, turning the body to the opposite side. The Supraspinales and 

 Interspinales by approximating the spinous processes help to extend the spine. 

 The Intertransversales approximate the transverse processes, and help to bend the 

 spine to one side. The Rotatores spinge assist the Multifidus spinge to rotate the 

 spine, so that the front of the trunk is turned to the side opposite to that from 

 which the muscle acts. 



Surface Forms. The surface forms produced by the muscles of the back are numerous and 

 difficult to analyze unless they are considered in systematic order. The most superficial layer, 



