272 THE PHILOSOPHY OF BIOLOGY 



The motor nerves are the efferent paths, the way out 

 from the central nervous system. 



The central nervous system is essentially the organ 

 for the integration of the activities of the whole body. 

 It is the " seat of multitudinous synapses," a description 

 which better than any other applies to the morphology 

 of the brain of the vertebrate animal. We have already 

 considered what is meant by the term " reflex action,'* 

 it is the series of processes which occur when a " reflex 

 arc " becomes functionally active. A reflex arc consists 

 of (i) a receptor organ, say a tactile corpuscle in the 

 skin ; (2) an afferent nerve fibre ; (3) a nerve cell in 

 the brain or spinal cord ; (4) an efferent nerve fibre ; 

 and (5) an effector nerve organ, say a motor plate in a 

 muscle fibre. The series of processes involved in a reflex 

 action consist of the stimulation of the receptor organ,, 

 the passage of the afferent impulse into the brain or cord, 

 the passage of the impulse through a series of cells in 

 the nerve centre forming a synapse, the transmission 

 of the impulse through the efferent nerve fibre into the 

 effector organ in the muscle and the stimulation of the 

 latter to an act of contraction. This is a purely 

 schematic description of the structures and processes 

 forming a reflex action and arc : in reality the path 

 both into and out from the central nervous system is 

 interrupted again and again, and at each place of 

 interruption there are alternative paths. The interrup- 

 tions occur at the synapses. At a synapse the nervous 

 impulse passes through an arborescence of fine nervous 

 twigs, into which the fibre breaks up, into a similar 

 arborescence, and these two arborescences are not in 

 actual physical contact : the impulse leaps over a gap. 

 At numerous places in both brain and cord there are 

 alternative synapses and at these places the impulse 

 may travel in more than one direction. 



