LOCALIZATION OF FUNCTION IN THE NERVOUS SYSTEM 115 



sible between all the special paths that lead from receptive areas 

 and these final common pathways to effector regions. The con- 

 duction of impulses according to this principle of the common 

 path furnishes the mechanism for the complicated and varied 

 responses that characterize nervous coordination. By means of 

 this mechanism the animal is enabled to behave as a unit in its 

 reactions to the numerous and changing conditions of its environ- 

 ment. 



Localization of Function in the Nervous System 



Up to this point, the analysis of nervous coordination has 

 been made in terms of the reflex arc. No particular emphasis has 

 been placed on the position of the neurones involved in these arcs 

 with reference to the morphology of the nervous sj-stem. It now 

 becomes interesting to understand the functions of the different 

 parts of the vertebrate nervous system. 



Peripheral Nervous System. — The peripheral nervous system 

 consists of spinal, cranial, and autonomic nerves. The general 

 function of these parts of the peripheral system is the transmission 

 of nervous impulses to the central .system from receptive regions, 

 and from the central system to the effector organs. In the case of 

 the spinal nerves, it was pointed out that processes of afferent 

 neurones entered the spinal cord over the dorsal roots of spinal 

 nerves, while the processes of efferent neurones passed out along 

 the ventral roots. Such nerves are called " mixed nerves " and 

 may be considered to represent the primitive condition of nerve 

 trunks. Certain of the cranial nerves, as the third or oculomotor, 

 also carry fibers of both sensory and motor neurones. Others 

 carry processes of but one type of neurone. The eighth cranial 

 nerve is made up entirely of processes of afferent neurones from the 

 auditory receptor; while the eleventh and twelfth cranial nerves, 

 found in the higher vertebrates, contain only efferent or motor 

 fibers to striated muscles. Finally, the autonomic nerves, which 

 include the sympathetic system, are entirely efferent, and con- 

 stitute the " final common paths " to glands and to the muscles 

 of the blood vessels and viscera. 



Central Nervous System. — The central nervous system is 

 composed of the spinal cord and brain. As has been repeatedly 

 imphed in the discussion of the reflex arc, the general function of 

 the central nervous system is the adjustment of incoming to out- 



