55o POPULAR SCIENCE MONTHLY. 



is a nervous arc, running from a receptor organ to an effector organ, 

 e. g., from a sense-organ to a limb-muscle. We may still, I think, 

 conveniently accept the morphological units termed neurones as units 

 of construction of the reflex arc. It may be that these neurones are 

 in some cases not unicellular, but pericellular. That question need 

 not detain us now. Accepting the neurone as the unit of structure of 

 the reflex chain, the characteristic of the synaptic system is that the 

 chain consists of neurones jointed together in such a way that conduc- 

 tion along the chain seems possible in one direction only. These junc- 

 tions of the neurones are conveniently termed synapses. The irrever- 

 sible direction of the conductivity along the neurone chain is probably 

 referable to its synapses. This irreciprocity of conduction especially 

 distinguishes the synaptic nervous system from the nerve-net system. 

 That neurone forms the sole avenue which impulses generated at its 

 receptive point can use whithersoever may be their distant destination. 

 That neurone is therefore a path exclusive to the impulses generated 

 at its own receptive points, and other receptive points than its own 

 can not employ it. 



But at the termination of every reflex arc we find a final neurone, 

 the ultimate conductive link to an effector organ, gland or muscle. 

 This last link in the chain, e. g., the motor neurone, differs obviously 

 in one important respect from the first link of the chain. It does not 

 subserve exclusively impulses generated at one single receptive source 

 alone, but receives impulses from many receptive sources situate in 

 many and various regions of the body. It is the sole path which all 

 impulses, no matter whence they come, must travel if they would reach 

 the muscle fibers which it joins. Therefore, while the receptive neu- 

 rone forms a private path exclusive for impulses of one source only, the 

 final or efferent neurone is, so to say, a public path, common to im- 

 pulses arising at any of many sources in a variety of receptive regions 

 of the body. The same effector organ stands in reflex connection not 

 only with many individual receptive points, but even with many vari- 

 ous receptive fields. Reflex arcs arising in manifold sense organs can 

 pour their influence into one and the same muscle. A limb muscle is 

 the terminus ad quern of nervous arcs arising not only in the right eye, 

 but in the left; not only in the eyes, but in the organs of smell and 

 hearing; not only in these, but in the geotropic labyrinth, in the skin 

 and in the muscles and joints of the limb itself and of the other limbs 

 as well. Its motor nerve is a path common to all these. 



Reflex arcs show, therefore, the general feature that the initial neu- 

 rone is a private path exclusive for a single receptive point; and that 

 finally the arcs embouch into a path leading to an effector organ, and 

 that this final path is common to all receptive points wheresoever they 

 may lie in the body, so long as they have any connection at all with 



