THE NERVOUS SYSTEM 185 



If the knees are crossed or if the leg is allowed to dangle freely 

 from a chair or table, a tap on the patellar ligament, just below 

 the knee cap, causes the foot to be jerked forward by the con- 

 traction of the quadriceps femoris muscle of the thigh. The 

 peripheral nerve connections involved are the second and third 

 lumbar spinal nerves, which provide the afferent pathways from 

 the skin and ligament to the spinal cord, and also the efferent 

 pathways from the spinal cord to the thigh muscle. In addition 

 there are central pathways consisting of intermediate neurons in 

 the spinal cord that form an integral part of the nervous circuit. 

 The entire nervous pathway along which an impulse is trans- 

 mitted to and from the central nervous system is a reflex arc. 

 In the cord or brain a sensory neuron may connect with several 

 motor neurons and its stimulation may thus bring about response 

 in a number of effectors. Intermediate neurons connecting 

 sensory and motor neurons of the same side are called association 

 fibers; if they connect sensory neurons of one side with motor 

 neurons on the opposite side, they are called commissural fibers. 

 Thus through commissural and association pathways sensory 

 impulses may affect motor neurons of the same or opposite side, 

 or of different levels. Reflexes occur not only in the central 

 nervous system but also in the peripheral ganglia of the auto- 

 nomic system. The reflex is thus the functional unit common to 

 all parts of the nervous system. 



Synapse. — The axon of a neuron may terminate in an effector, 

 such as muscle, or it may connect with the dendrites of another 

 neuron. Whether the connection between two neurons is merely 

 one of contact or actual fusion is a question about which much 

 has been written. The term synapse refers to the functional 

 connection between two neurons by means of which the impulse 

 is transferred from one to the other. 



Functional Significance of the Central Nervous System. — The 

 cell bodies of the sensory neurons are located in the spinal ganglia, 

 in the ganglia of the cranial nerves, in the olfactory epithelium, 

 and in the retina of the eye. The sensory fibers make connections 

 through intermediate neurons with motor neurons. The brain 

 and spinal cord are composed of tracts or groups of neurons, all 

 of the fibers of a tract usually having similar functions. In sim- 

 ple reflexes, the cord alone may be involved in distributing sen- 

 sory impulses to motor neurons. In other cases the sensory 



