THE FUNCTIONAL UNIT OF THE NERVOUS SYSTEM 577 



system. Physiologically, however, the neuron attains its greatest 

 importance only when several of them are joined to form reflex circuits, 

 because only then do we obtain the structural basis for the reflex act 

 which constitutes the functional unit of the nervous system. Obviously, 

 if an electric shock is passed directly into muscle tissue, it reacts by 

 giving a contraction. The same result may be obtained by stimulat- 

 ing the nerve innervating this muscle. In either case, it is to be 

 noted that this action does not constitute a reflex, because it is accom- 

 plished in a direct manner and not through the intervention of a num- 

 ber of neurons arranged in proper series. In order that the aforesaid 

 muscular contraction may become a true reflex response, it is neces- 

 sary to bring the stimulus to bear upon some afferent nerve, whence 

 the impulse is transferred to the motor nerve of this muscle. 



Clearly, the cells constituting the tissues and organs of the higher 

 forms, behave in the same manner as unicellular organisms. They 

 possess irritability, conductivity and contractility and hence, give 

 rise to motor effects whenever stimulated. If a vorticella is touched, 

 an excitation results which is conducted to the myoids situated in its 

 stalk. A contraction follows which causes the bell-shaped upper 

 portion of this organism to be retracted from the seat of the stimula- 

 tion. In a similar way, an electrical shock applied to a muscle, 

 gives rise to a wave of excitation which finally leads to general changes 

 within its myoplasm. The function of the nervous system, there- 

 fore, is not to impart these elementary properties to organisms, be- 

 cause all living substance is irritable, conductile and contractile. 

 Its real object is to insure a functional correlation between the different 

 cellular units of the body, so that the latter are enabled to react to 

 changes in the environment as one single coordinated whole. It is 

 also true that nervous tissue is peculiarly suited to bring this coopera- 

 tion about, because the neuroplasm of which it is composed, possesses 

 the properties of irritability and conductivity in an even greater 

 measure than ordinary living substance. 



A general survey of the animal kingdom shows that the forms be- 

 low the ccelenterata do not possess definite nervous structures. 

 Their life processes, as far as we know at the present time, are not 

 correlated by cells other than those forming their tissues. In the 

 ccelenterata, however, certain cells are found which are particularly 

 sensitive and appear to be set aside for the singular purpose of receiving 

 stimuli from without and of transferring the resulting impulses to 

 other colonies of cells. We find these units in the external strata of 

 the body, i.e., in the epiblast (Fig. 287, A). Their internal poles are 

 drawn out into slender processes which eventually invade the deeper 

 layers (Fig. 287, B and C). Here they are brought into contact with 

 secondary nervous elements which finally connect with the underlying 

 muscle tissue (Fig. 287, D). An arrangement of this kind, representing 

 really the lowest type of nervous system, is found in the jelly-fish. 

 The sensory cells which are situated in among the external lining cells 



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