Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 



285 



Fig. 16.7. A nerve from a kitten's tongue. A, Cross section showing the cut ends 

 of nerve cell fibers. In each one the dark center is the axon; the pale ring around 

 it is the sheath. The nerve is enclosed in a sheath of connective tissue. B, Longi- 

 tudinal section. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. 

 New York, McGraw-Hill Book Co., Inc., 1953.) 



the vertebrates ganglia are prominent in the dorsal roots of the spinal nerves. 

 Nerves are bundles of nerve cell fibers that convey sensory and motor 

 impulses between the brain and spinal cord and other parts of the body. 

 Sensory nerves contain fibers that conduct impulses from the sense receptors 

 to the cord or brain, e.g., the optic nerve from the eye. Motor nerves contain 

 fibers that conduct impulses from the brain and cord to muscles or glands. 

 The trunks of the spinal nerves contain both sensory and motor fibers, as do 

 some of the cranial nerves (Table 16.1). 



Conduction — the Nerve Impulse 



The nerve impulse is not yet understood. The statements that follow may 

 apply to it as a whole or only to a process which accompanies it. 



The nerve impulse is an electrochemical process that passes through a 

 neuron. It represents conduction at its highest development and speed. The 

 impulse enters the cell through the dendrites and passes through the cell 



