50 A COURSE ON ZOOLOGY. 



form the brachial plexus, whose branches are distributed 

 to the arms. The dorsal nerves are distributed to the 

 trunk, and are very simple in their directions, having no 

 plexus. The lumbar nerves, on the contrary, all unite 

 to form a large and deeply-placed plexus, called the 

 lumbar plexus. Lastly, from the first pairs of sacral 

 nerves is formed the sacral plexus, which terminates in 

 a flattened trunk, the great sciatic nerve, and this is 

 both the thickest and the longest nerve in the body. It 

 supplies the lower limb, sending branches to all parts of 

 the leg and foot. 



The principal nerves are generally named after the 

 parts in which they are situated. Thus, there is a crural 

 nerve, a brachial nerve, a cubital nerve, etc. By fol- 

 lowing out in minute examination the nerves as they 

 pass to the organs, it may be seen that the last filaments 

 of motor nerves penetrate into the muscular tissue, while 

 those of the nerves of general sensation terminate in 

 the thickness of the skin. 



Great Sympathetic System. The great sympathetic 

 system is composed of ganglions and nerves, the gan- 

 glions being nerve-centres. The ganglions in the neck 

 and thorax are arranged in a regular manner in pairs 

 along the spinal column. In the 'other parts of the 

 system they are scattered, being found in the neigh- 

 borhood of all the organs, or even in the organs them- 

 selves. 



The nerves that make up this system form numerous 

 plexi, the plexi appearing at frequent intervals and in 

 the most complicated forms. Each organ has an impor- 

 tant plexus : thus there is a cardiac plexus, a mesenteric 

 plexus, a renal plexus, a hypogastric plexus. The last is 

 situated, as its name indicates, in the upper part of the 



