Nervous System and Sense Organs 195 



are made between axons and dendrites and are known as synapses. An 

 impulse can pass in but a single direction over this synapse, and there is 

 normally some slowing of the rate of conduction at this point. Nearly 

 every nerve cell possesses several synapses with other cells. 



Although the processes of the neuron may extend to all parts of the 

 body, the nerve cell bodies are concentrated either in the central nerv- 

 ous system or in masses known as ganglia. The masses of cell bodies 

 are known as gray matter from their generally grayish appearance. 

 The processes are usually grouped into bundles known as nerves when 

 outside the central nervous system or as yierve pathways within the 

 brain and spinal cord. In the brain and spinal cord, these masses of 

 processes are known as white matter in contrast to the cell bodies or 

 gray matter. 



The individual processes may be either plain or covered with a 

 sheath of fatty tissue, the myelin sheath, which in turn is covered by 

 the neurolemma, a thin membrane. The nerves with the myelin sheaths 

 are known as myelinated nerves. The myelin sheath appears to act as 

 an insulator, and those nerves having this sheath conduct impulses more 

 rapidly than do those without it. Occasional breaks, the nodes of 

 Ranvier, occur in the myelin sheath. It has been suggested that these 

 nodes also serve to increase the speed of nerve impulse conduction. 



Many of the efferent fibers or axons innervate the skeletal muscles 

 where they initiate muscular contractions. As the fiber enters the muscle 

 tissue, the myelin sheath is lost. At the junction between the nerve 

 and muscle fibers the sarcoplasm of the muscle forms a mass beneath 

 the sarcolemma. This is the motor plate which receives the naked axon. 

 This axon breaks up into numerous ramifications. The endings of the 

 afferent fibers are usually less specialized than this. Often they are 

 spindle-shaped structures, and occasionally they are enclosed within cap- 

 sules. Those afferent fibers which function as sense organs are more 

 complex in structure. 



The Functional Unit of the Nervous System. — Inasmuch as it 

 is very nearly impossible to discuss the anatomy of the nervous system 

 without referring to its function, it is best to describe briefly the func- 

 tional unit of the nervous system, the reflex arc (Fig. 58). In its sim- 

 plest form, this reflex arc consists of an afferent neuron carrying an im- 

 pulse in from a sense organ to the spinal cord, an association neuron 

 carrying the impulse across in the cord to the efferent neuron, and the 

 efferent neuron which carries the impulse to the effector muscle. It 



