58 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



reflex; by center they may be called cerebral and spinal 

 or axial, for it includes the medulla as well as the cord. 

 The center of the one is the surface gray matter of the 

 cerebrum ; the center of the other the central gray matter 

 of the cord and its continuation in the skull. Each sub- 

 system has its sensory or afferent and its motor or ef- 

 ferent fibers, but the two subsystems are so closely 

 connected that they may act as one. The spinal nerves 

 carry both kinds of fibers, which may act as belonging 

 to both systems. The cranial nerves usually carry but 

 one kind — i. e., either sensory or motor, acting for both 

 systems. 



Course and Termination of Fibers. — A sensory fiber of 

 the cerebral system, beginning in a sensory cell of the 

 cerebral cortex, passes down a posterior column of the 

 cord, communicates with a sensory cell of a posterior 

 cornu, and, continuing, becomes a sensory fiber of the 

 reflex system as well as the cerebral system, and then 

 goes out by a posterior root of a spinal nerve to termi- 

 nate in a sensitive surface or a sense organ. A motor 

 fiber of the same system goes from a motor cell of the 

 cerebral cortex, down an anterior column of the cord, 

 communicates with a motor cell of the anterior cornu, 

 and continues as a motor fiber of both systems, to termi- 

 nate in a muscle. The sensory and motor cells, both of 

 the cerebrum and of the spinal cord, connect with one 

 another, so as to complete the circuit, of the cerebral 

 system in the one case, and of the spinal system in the 

 other. 



Or, more explicitly, and tracing each impulse in the 

 direction of its transmission : A sensory fiber of the 

 cerebral system, commencing in a terminal on a sensi- 

 tive surface or in a sense organ, passes up a spinal 

 nerve, through a posterior root into a posterior cornu, 

 communicates there with a spinal sensory cell, then goes 



