136 THE HUMAN BODY. 



pass from the spinal canal, the posterior roots form a ganglion 

 on each side to which the anterior roots unite themselves, 

 and from which the nerve is distributed to the organism, by 

 three classes of spinal branches, anterior ', posterior , and gan- 

 glionic; the latter unite with the great sympathetic. 



The first four pairs of cervical nerves form by the con- 

 tiguity of their branches the cervical plexus ; the ramifications 

 of which distribute themselves to the surface, and to the 

 deep portions of the neck, to the outside of the head, to the 

 shoulder, and to the upper portion of the back. 



The last four pairs constitute the brachial plexus, which, 

 after furnishing numerous branches to the shoulder and back, 

 go to the arm as the brachio-cutaneous, musculo-cutaneous, 

 median, radial, and ulnar nerves. 



The twelve pairs of dorsal or intercostal nerves, as well as 

 the five pairs of lumbar nerves, are ramified in the walls of 

 the thorax and abdomen, and in the muscles of the back and 

 loins. The lumbar plexus furnishes, among other principal 

 branches, the crural nerve, which ramifies into the musculo- 

 cutaneous of the leg, and the external and internal saphenous 

 nerves, &c. 



The six pairs of sacral nerves are distributed to the pelvis 

 and to the lower limbs. The first four pairs with the last 

 lumbar pair form the sacral plexus, the principal terminal 

 branch of which is the sciatic nerve. This is the largest 

 nerve in the body; it descends through the posterior portion 

 of the thigh, to the muscles of which it furnishes several 

 branches; and a little above the knee it divides into two 

 trunks the internal popliteal we tibial, and the external or 

 peroneal, which distribute themselves by numerous ramifica- 

 tions to the muscles of the leg and foot. 



The great sympathetic. The nervous apparatus designated 

 by this name consists of a double cord placed on either side 

 of the spinal column, the whole length of the neck, and in 

 the interior of the thoracic and abdominal cavities. It is, as 

 has ajready been stated, the nervous system of organic, vege- 

 tative, or nutritive life. Extending from the first cervical 

 vertebra to the last vertebra of the sacrum, the great sym- 

 pathetic enlarges at the level of each vertebra, and forms 



