THE SUBOCCIPITAL MUSCLES. 319 



capitis posticus major muscle arises by a narrow tendon from the spinous process 

 of the axis, and expanding as it passes upwards and outwards, is inserted into and 

 below the outer part of the inferior curved line of the occipital bone Its insertion 

 is inside and beneath that of the superior oblique muscle. 



The rectus capitis posticus minor muscle arises from the posterior arch of 

 the atlas by the side of the tubercle, and expands as it passes upwards to be inserted 

 into the inner third of the inferior curved line of the occipital bone, and the depres- 

 sion between that and the foramen magnum. 



The obliquus capitis inferior, the strongest of the muscles now under con- 

 sideration, arises from the spinous process of the axis, between the origin of the 

 rectus posticus major and the insertion of the semispinalis colli, and is inserted into 

 the lower and back part of the transverse process of the atlas. 



The obliquus capitis superior, smaller than the preceding muscle, arises from 

 the upper surface of the transverse process of the atlas, thence inclines obliquely 

 upwards and backwards, increasing in breadth as it ascends, and is inserted into an 

 impression between the outer parts of the curved lines of the occipital bone. 



Relations. The two oblique muscles with the rectus major form the sides of a small 

 triangular space the suboccipital triangle, in the area of which the posterior primary branch 

 of the suboccipital nerve and the vertebral artery are found. 



Varieties of the suboccipital muscles. Longitudinal division or doubling of the rectus 

 posticus major or minor is not unfrequent ; doubling of the oblique muscles is less common. 

 Accessory slips are sometimes found passing in continuation of the spinalis cervicis from the 

 spines of one or two of the cervical vertebrae and the ligamentum nuchas to the occipital bone. 

 The atlanto-ma-xtoid is a small muscle often present, arising from the transverse process of the 

 atlas between the obliquus superior and rectus lateralis, and inserted into the hinder part of 

 the mastoid process. A supernumerary muscle has also been seen passing from the spine of 

 the axis to the mastoid process. 



Nerves. All the muscles of the back which act upon the head and spine, viz., the splenius. 

 erector spinae, complexus, and the muscles more deeply seated are supplied by the posterior 

 primary branches of the spinal nerves. 



Actions of the dorsal spinal and cranial muscles. In extending the spinal column and 

 head not only the erector spines, but all the muscles of this group come into play, necessarily 

 acting simultaneously on both sides. Acting on one side alone they produce the lateral flexion 

 of the column. While the action of the longitudinally directed muscles is limited to these 

 movements, the oblique muscles are farther enabled to rotate the head and spinal column. 

 Thus, when the sple-nius of one side acts it rotates the head and neck to the same side, while 

 the complexus and transvero-fpinalet rotate the head and spine to the opposite side. The 

 power of the extensor muscles to straighten the back from the flexed condition, as measured 

 by the muscular dynamometer, varies in adults of medium strength from 2001bs. to 4001bs. 

 A certain amount of extension of the spine, as will be hereafter explained, accompanies 

 inspiration ; but if the spine be fixed, some of the erector muscles may, by their costal attach- 

 ments, depress the ribs, and thus assist in forced expiration. 



Of the short cranio-vertebral muscles, two the rectus minor and superior oblique act 

 principally by drawing the head backwards, that being the chief movement allowed between 

 the atlas and occipital bone ; while the principal action of the rectus posticus major and the 

 inferior oblique, when acting on one side, is to rotate the atlas and skull upon the axis, the 

 former muscle also assisting in the extension of the head. 



THE MUSCLES OF THE THORAX, 



The muscles of the thoracic wall are the intercostals, levatores costarum, sub- 

 costals, and triangularis sterni, and along with these the diaphragm intervening 

 between the thorax and abdomen may conveniently be grouped. 



The INTERCOSTAL MUSCLES consist of two thin layers of short oblique fibres 

 occupying the intercostal spaces : these layers are named respectively the external 

 and internal muscles. 



The external intercostal muscles are stronger than the internal, and thicker 

 behind than in front. Their fibres are intermixed with a considerable quantity of 



