828 THE NERVOUS SYSTEM. 



limits variable, and of course does not correspond to the point of emergence of 

 the nerve from the intervertebral foramina. The preceding table, from Mac- 

 alister, shows as accurately as can be shown the relation of these points of origin 

 from the spinal cord to the bodies and spinous processes of the vertebrae. 



THE CERVICAL NERVES. 



The roots of the cervical nerves increase in size from the first to the fifth, and 

 then remain the same size to the eighth. The posterior roots bear a proportion 

 to the anterior as 3 to 1, which is much greater than in any other region, the 

 individual filaments being also much larger than those of the anterior roots. In 

 direction the roots of the cervical are less oblique than those of the other spinal 

 nerves. The first cervical nerve is directed a little upward and outward ; the 

 second is horizontal ; the others are directed obliquely downward and outward, 

 the lowest being the most oblique, and consequently longer than the upper, the 

 distance between their place of origin and their point of exit from the spinal canal 

 never exceeding the depth of one vertebra. 



The trunk of the first cervical nerve (suboccipitaT) leaves the spinal canal between 

 the occipital bone and the posterior arch of the atlas ; the second, between the 

 posterior arch of the atlas and the lamina of the axis ; and the eighth (the last), 

 between the last cervical and first dorsal vertebrae. 



Each nerve, at its exit from the intervertebral foramen, divides into a posterior 

 and an anterior division. The anterior divisions of the four upper cervical nerves . 

 form the cervical plexus. The anterior divisions of the four lower cervical nerves, 

 together with the first dorsal, form the brachial plexus. 



Posterior Divisions of the Cervical Nerves (Fig. 495). 



The posterior division of the first cervical (subocdpitaT) nerve differs from the 

 posterior divisions of the other cervical nerves in not dividing into an internal 

 and external branch. It is larger than the anterior division, and escapes from the 

 spinal canal between the occipital bone and the posterior arch of the atlas, lying 

 behind the vertebral artery./ It enters the suboccipital triangle formed by the 

 Rectus capitis posticus major, the Obliquus superior, and Obliquus inferior, and 

 supplies the Recti and Obliqui muscles, and the Complexus. From the branch 

 which supplies the Inferior oblique a filament is given off which joins the second 

 cervical nerve. This nerve also occasionally gives off a cutaneous filament, which 

 accompanies the occipital artery and communicates with the occipitalis major and 

 minor nerves. 



The posterior division of the second cervical nerve is three or four times greater 

 than the anterior division, and the largest of all the posterior cervical divisions. 

 It emerges from the spinal canal between the posterior arch of the atlas and 

 lamina of the axis, below the Inferior oblique. It supplies this muscle, and 

 receives a communicating filament from the first cervical. It then divides into an 

 internal and external branch. 



The internal branch, called, from its size and distribution, the occipitalis major, 

 ascends obliquely inward between the Obliquus inferior and Complexus, and pierces 

 the latter muscle and the Trapezius near their attachments to the cranium. It is now 

 joined by a filament (third occipital) from the posterior division of the third cervical 

 nerve, and, ascending on the back part of the head with the occipital artery, di- 

 vides into two branches, which supply the integument of the scalp as far forward 

 as the vertex, communicating with the occipitalis minor. It gives off an auricular 

 branch to the back part of the ear and muscular branches to the Complexus. 

 The external branch is often joined by the external branch of the posterior 

 division of the third, and supplies the Splenius, Trachelo-mastoid, and Complexus. 



The posterior division of the third cervical is smaller than the preceding, but 

 larger than the fourth ; it differs from the posterior divisions of the remaining 

 cervical nerves in its supplying an additional filament, the third occipital nerve, 



