3- THK NKUYOl S S^STI.M AM) ITS <J< >\SKI; VATIOX 



gray matter, this could only result in confusion of 

 re.-iction and, in case sensations were aroused, in 

 hallucinations. This assertion can be justified later 



Fig. li). 

 Let us consider in what various ways nerve-fibers 



may terminate. Those which serve the receptors are 

 c:dled nfit r< nt, tliose wliich act on effectors are said to he 

 <i]'n-niL Most afferent fibers are parts of neurons like 

 those described in the paragraph above; the perikaryon 

 in each one is neither at the beginning nor at the end of 

 the course traversed by the impulses. It is most often 

 placed on what might be called a "spur-track," nearer the 

 central than the peripheral limit of the neuron. It was 

 long a question whether the passing impulses entered the 

 perikaryon or left it quite at one side as they ran inward. 

 There is some reason to think that they actually visit it. 

 By a rather arbitrary stretching of our terms we may 

 regard the section of such a neuron that lies between the 

 periphery and the perikaryon as an elongated dendrite, 

 ;md the portion central to the perikaryon as the axon. 

 But it is not customary to call a fully developed and 

 ensheathed nerve-fiber a dendrite, and this is the character 

 of the conductor in question. 



When we think of the limits of neurons in an anatomic 

 sense it is well for us always to have their physiologic 

 action in mind. If we do so we can speak intelligibly of 

 I >c<l innings and rm//m/x, the first being the receptive 

 processes, and the second the terminals which transmit 

 effects to other neurons or to tissues not belonging to 

 the nervoiH sy-tem at all. The afferent or receptor neu- 

 rons which we have been discussing hci/in, in this sense, 

 where they lie exposed to stimulation by pressure, heat 

 or cold, or chemical agencies. They cm! where they 

 form synap-es with other neurons in the gray matter of 

 the central axis. To refer to beginnings and endings 

 with this meaning is an entirely different thing from using 

 the same expressions with respect to origin and growth. 

 \\ e cannol deal here with the einbryologic facts. 



