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THE NERVOUS SYSTEM. 



area, comprising the chief viscera ; this area is governed by the sympathetic system, 

 subordinate to and controlled by its connexions with the splanchnic or visceral 

 branches of the spinal nerves. 



The cerebral nerves are twelve in number (see note, p. 798), arranged in 

 pairs ; they present striking differences in origin, in distribution, and in functions. 



The spinal nerves are usually thirty-one in number, also arranged in pairs. 

 Each nerve arises by two roots from the spinal medulla, one posterior and gangliated, 

 the other anterior and not gangliated. After each root has pierced separately the 

 dura mater, the two roots become enclosed in a common sheath, and unite to form 

 the spinal nerve in the intervertebral foramen ; emerging from this, the nerve is 

 distributed to the trunk and limbs in a manner to be described later. 



The nerves are designated cervical, thoracic, lumbar, sacral, and coccygeal, in rela- 

 tion to the vertebrae between which they emerge from the vertebral canal. Each 

 nerve appears above the corresponding vertebra, in the cervical region, except the 

 eighth, and below the corresponding vertebra in all other regions. There are thus 

 eight cervical nerves (the last appearing between the seventh cervical and first 

 thoracic vertebrae) ; there are twelve thoracic, Jive lumbar, five sacral, and one 

 coccygeal nerve, all appearing below the corresponding vertebrae. 



The thirty-first nerve is occasionally absent; and there are sometimes one or two 

 additional pairs of minute filaments below the thirty -first, which, however, do not 

 emerge from the vertebral canal. These are rudimentary caudal nerves. 



The size of the spinal nerves varies. The largest are those which take part in the 

 formation of the great nerve -trunks of the limbs (lower cervical and first thoracic, 

 and lower lumbar and upper sacral nerves) ; and of these the nerves destined for the 

 lower limbs are the larger. The coccygeal nerve is the smallest of the spinal nerves; 

 tbe thoracic nerves (except the first) are more slender than the limb nerves ; and 

 the cervical nerves diminish in size from below upwards. 



Systema Sympathicum. The sympathetic system consists of a pair of gangliated 

 trunks, connected, on the one hand, in. certain regions to the spinal nervous system by a 

 series of white rami communicantes splanchnic or visceral branches of the spinal nerves ; 

 and, on the other hand, distributing branches (a) to the spinal nerves (gray rami com- 

 municantes), and (b) to the viscera and vessels occupying the splanchnic area. The 



