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THE NERVOUS SYSTEM. 



The posterior divisions of the spinal nerves are generally smaller than the 

 anterior ; they arise from the trunk resulting from the union of the roots, in the 

 intervertebral foramina ; and, passing backward, divide into internal and external 

 branches, which are distributed to the muscles and integument behind the spine. 

 The first cervical, the fourth and fifth sacral, and the coccygeal, do not divide 

 into external and internal branches. 



The anterior divisions of the spinal nerves supply the parts of the body in 

 front of the spine, including the limbs. They are for the most part larger than 

 the posterior divisions. Each division is connected by a slender filament with the 

 sympathetic. In the dorsal region the anterior divisions of the spinal nerves are 

 quite separate from each other, and are uniform in their distribution ; but in the 

 cervical, lumbar, and sacral regions they form intricate plexuses previous to their 

 distribution. 



Points of Emergence of the Spinal Nerves. 



The roots of the spinal nerves from their origin in the cord run obliquely 

 downward to their point of exit from the intervertebral foramina, the amount of 

 obliquity varying in different regions of the spine, and being greater in the lower 

 than the upper part. The level of their emergence from the cord is within certain. 



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. The 

 posterior root of the first cervical is an exception to this rule : it is smaller than 

 the anterior root. 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 up- 

 ward 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 (suboccipitafy leaves the spinal canal between 

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



