474 COMPARATIVE ANATOMY 



cells are in protoplasmic continuity with one another, and nerve impulses 

 are carried directly from cell to cell. The differentiated neurons of 

 higher animals appear not to be so interconnected, but have greater 

 individuality, for the telodendria of one neuron are brought into relation 

 with the dendrites of another only indirectly through the so-called synapse. 

 A synapse is the region where the fine telodendria of one neuron are 

 brought into physiological relation with the dendrites of another neuron. 

 Nerve impulses which involve the activity of two neurons must pass 

 through such a synapse. The transmission of a nervous impulse through 

 the synapse is believed to involve a semipermeable membrane through 

 which impulses pass when conveyed from one neuron to another. The 

 physical process by which such a transfer is effected has been compared 

 to the jump-spark action of a gasoline engine. 



It should not, however, be understood that this synaptic membrane 

 has been demonstrated beyond a reasonable doubt. Its presence is 

 inferred chiefly because refined neurological technique has not been able 

 to demonstrate the continuity of the neurofibrillae of adjacent neurons. 

 The fact that each neuron arises from a neuroblastic cell which is primarily 

 independent of other cells and that the termination of a growing neurite 

 is free further strengthens this conclusion. Some physiological experi- 

 mental evidence points in the same direction. 



With the differentiation of the neuron, with neurite and dendrites 

 which normally convey nerve impulses in one direction only, and with 

 tigroid bodies and neurofibrillae in its cytoplasm, the evolution of the 

 nervous unit or neuron reaches its climax. 



The steps in the phylogenesis of the nervous system are, therefore, the 

 differentiation of the neurosensory cell, the attainment of functional con- 

 nexion with muscle or gland cells, the recession of the neurosensory cell 

 from the external epithelium to form a primitive ganglion cell, the forma- 

 tion of an interconnected nerve-net containing association cells, the union 

 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 neurones and their interconnexions within a 

 nerve-net or nerve center. 



