808 



THE NERVOUS SYSTEM 



sumes a distinct morphological separation at the synapse, but the reticular 

 theory postulates a direct morphological continuity. We shall assume that the 

 neuron doctrine is correct. 



b. The Reflex Arc 



While the neuron, in a strict sense, represents the functional unit of the 

 nervous system, in reality, chains of physiologically related neurons form the 

 functional reflex mechanism of the vertebrate nervous system. The functional 



SPINAL CORD 

 TELODENDRIfl 



DENDRITES 



DENDRITES 



SENSE ORGAN 

 OR 

 RECEPTOR ORGAN 



'/id' 



'0/ MUSCLE FIBER OR 



^ EFFECTOR ORGAN 



MYELIN SHEATH-==4^^%--4^* 



NEURILEMMA CELL ^ \-A'^%'-Kml*i 



PROTOPLASMIC NET 

 AXIS CYLINDER 



NODE OF RANVIER 

 SHEATH CELL 



Fig. 352. Neuron structure and relationships. (A) Structural components of a simple 

 reflex arc. (B) Diagrammatic representation of a motor neuron. (Redrawn from 

 Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders, after 

 Barker.) (C) Developing nerve fiber (process) of young neuroblast. Observe growth 

 or incremental cone at distal end of growing process. (Redrawn from Ranson, 1939, 

 The Anatomy of the Nervous System, Philadelphia, Saunders, after Cajal, Prentiss-Arey.) 

 (D) Neuron from spinal ganglion of a dog showing ganglion cell body with its sur- 

 rounding capsular cells and capsule. Observe that the capsular cells and capsule are 

 continuous with sheath cell and neurilemma. (Redrawn, somewhat modified, from 

 Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders.) (E) 

 Longitudinal section of myelinated nerve fiber. (Redrawn from Ranson, 1939, The 

 Anatomy of the Nervous System, Philadelphia, Saunders, after Nemiloff, Maximow- 

 Bloom.) 



