THE NERVOUS SYSTEM 



345 



of afferent and efferent neurites into bundles or nerves, and the concen- 

 tration of nerve-cell bodies to form ganglia. 



ORGANIZATION OF THE NERVOUS SYSTEM 



When nervous units convey impulses towards and away from a subcu- 

 taneous nerve-net, as in the coelenterates, there are the beginnings of a 

 nervous system. The nerve-net forms the central nervous system, the 

 afferent and efferent neurites the peripheral nervous system. The primi- 

 tive and characteristic function of such a system is the nervous reflex. 

 A nervous reflex, or reflex action, is a simple motor response to stimulus 

 involving sensory and motor neurons and their interconnexions within a 

 nerve-net or nerve center. 



If we take the subcutaneous nerve-net of coelenterates as an early 

 stage in the evolution of the central nervous system and the neurosensory 



A. LOWER njVTWORM B HIGHER FLATWORM C. ARTHROPOD 0. VERTEBRATE 



Fig. 306. — Diagrams of the nervous system of ^, lower flatworm, B, higher 

 flatworm, C, arthropod, and D, vertebrates. The higher nerve centers are cross- 

 hatched. The sympathetic cords are indicated by dotted Hnes. Many morphologists 

 assume that the figures represent a phylogenetic series. (Redrawn after Stempell.) 



cells as the beginnings of sensory nerves, the primitive ganglion cells are 

 the original association and motor cells. From such beginnings it is not 

 difficult to derive the complex nervous systems of the higher animals. 



The flatworms show a distinct advance above the coelenterate stage. 

 In them, the nerve-net is partly aggregated into two or more paired longi- 

 tudinal cords or connectives, which unite at the anterior end of the worm, 

 in close association with pigmented eye-spots, to form the beginnings of a 

 brain. (Fig. 306) 



Morphologists are inclined to derive the paired lateral nerve cords of 

 flatworms directly from the subumbrellar gangUonic ring of a medusa. 

 Like the subumbrellar ring, the nerve cords of flatworms consist of nerve 

 fibers associated with clusters of primitive ganglion cells. (Fig. 307) 

 Kappers explains the concentration of nervous material in the anterior 

 brain as a result of the great exposure of the head to stimuli. Unfortu- 

 nately, such an hypothesis is unsupported by experimental evidence. 



