xvi THE NERVOUS SYSTEM 223 



All nerve tissue consists of such cells completely sur- 

 rounded and held together by connective tissue and fat. 

 These substances protect, pack in, and hold in place the 

 delicate nerve tissue and enable it to carry on its func- 

 tions properly and easily. The entire nervous system is 

 made up of nerve cells and connective tissue. 



Take a bit of nerve tissue and examine it under the microscope. 

 Notice the size and shape of nerve cells. 



155. The Nerves. We saw that there are two kinds 

 of nerves necessary, one leading to and the other from 

 the brain. The former are called sensory or afferent, 

 and the latter motor or efferent. Many of the nerves 

 do not go directly to the brain, but reach it through the 

 spinal cord, and for that reason are known as spinal 

 nerves. There are tJiirty-one pairs of this kind. They 

 carry stimuli from and send impulses to the muscles 

 and glands of the trunk and the extremities. Those 

 connected directly with the brain are called cranial 

 nerves. TJiere are twelve pairs of cranial nerves. The 

 cranial nerves are distributed to the head and to the 

 internal organs. All of the special senses except touch 

 or feeling have this class of nerves coming from them. 

 The olfactory of the nose, the optic of the eye, and the 

 auditory of the ear are examples of cranial nerves. 



In their appearance all nerves are quite alike. They 

 are smooth, white, shining cords of various sizes and 

 lengths. The great sciatic nerve of each lower limb is 

 about one half of an inch wide, one sixth of an inch in 

 thickness, and extends from the lower end of the spinal 

 cord down to the toes, sending off branches all along its 



