THE BEAIX AND XERVES. " 405 



source of action. It is invested with three membranes, the 

 outer one being very dense and tough, and attached to the 

 inside of the skull. The second membrane lies close to the 

 brain, and is attached to it by many little blood-vessels. 

 The third is a very fine substance, like spider's web. 



The sj)hial cord is a continuation of the medulla oblon- 

 gata back through the whole length of the spinal canal. It 

 is commonly, but improperly, called the spinal marrow. It 

 is invested by the same membranes that the brain is. 



The nerves are round cords which arise in pairs, or by 

 tw^os, from the brain and spinal cord, and which connect 

 them with all other parts of the animal. There are forty- 

 six pairs of nerves. Of these, ten pairs arise within the 

 skull and thirty-six from the spinal cord. The nerves are 

 composed of similar substance to the brain, and invested by 

 a tough membrane. Most nerves divide into many branches, 

 like blood-vessels, until the branches become so fine as to 

 only be seen by the aid of the microscope. 



Of those that arise within the skull, 



The Jirst imir, or olfactory nerves, are distributed over the in- 

 side of the nose. They are the nerves of the sense of smell. 



The second jpair, or optic nerves, pass, one to the back 

 part of each eye. They are the nerves of sight. 



The third pair, or movers of the eyes, pass to the muscles 

 of the eyes, and influence their motions. 



The fourth pair pass to the superior oblique muscle of the 

 eyes. • 



The fifth pair have many divisions distributed to many 

 parts of the face, head, jaws, and inside parts of the mouth. 

 One branch of these nerves goes to the tongue, and gives 

 rise to the sense of taste. 



The sixth pair pass to some muscles of the eye. 



The seventh pair are distributed chiefly to the ear, and are 

 the nerves of the sense of hearing. They are called audi- 

 tory nerves. 



