54 



THE NERVOUS SYSTEM 



includes the entire motor neuron, with its cell body in the anterior gray column 

 and its motor ending on the muscle, and (5) the effector or organ of response, 

 which in this case is a muscle-fiber. A wave of activation, known as the nerve 

 impulse, is developed in the sensitive receptor, travels over this arc, and on 

 reaching the muscle causes it to contract. So simple a reflex is rare, but prob- 

 ably the knee-jerk is an example (Jolly, 1911). A more common form of reflex 

 arc involves a third, and purely central neuron, as illustrated on the right side 

 of Fig. 31. Such central elements may be spoken of as association and com- 

 missural neurons. Many of them serve to connect distant parts of the central 



Fig. 32. Diagram representing some of the conduction paths through the mammalian central 

 nervous system. An elaborate system of central or association neurons furnishes a number of 

 alternative paths between the primary sensory and motor neurons. (Redrawn from Bayliss.) 



nervous system with each other (Fig. 68). It is to the multiplication of these 

 central neurons that we owe the complicated pathways within the mammalian 

 brain and spinal cord. 



Pathways Through Higher Centers. A good idea of how the neurons of some 

 of the centers in the brain are related to the primary motor and sensory spinal 

 neurons is given by Fig. 32. It will be seen that many paths are open to an 

 impulse entering the spinal cord by way of a dorsal root fiber: (1) It may pass 

 by way of a collateral to a primary motor neuron in a two-neuron reflex arc. 

 It may travel over an association neuron, belonging (2) to the same level of the 



