344 CHORDATE ANATOMY 



The first nervous connexions appear to have been between skin and 

 muscle, by means of neurosensory cells, so that a motor response to 

 external stimulus is made possible. So simple an arrangement as this, 

 however, is rarely found, even in worms and molluscs. Usually at least 

 two nerve cells are involved in a reflex action, a neurosensory receptor and 

 a motor ganglion cell which connects with muscle or gland. Even more 

 frequently, a third or association cell is interpolated between the receptor 

 and the motor cells. These association cells may multiply to form a chain 

 of neurons within the central nervous system. Such complications were 

 made possible by the genesis of the central nervous systems of higher 

 animals from the nerve-net of coelenterates. (Fig. 305) 



In the primitive nerve plexuses of lower animals such as the coelen- 

 terates, and also in those of the vertebrate aUmentary canal, the ganglion 

 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 sjmapse. 

 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 beUeved to involve a semipermeable membrane through 

 which impulses pass 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 a neuron having 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 



