CH. XLIX.] THE COMMON PATH 717 



of conducting paths, which is called the central nervous system. So 

 numerous are the potential connections in this labyrinth that the 

 impulse may, under such abnormal conditions as strychnine poison- 

 ing, radiate in all directions, and be discharged so as to throw all the 

 muscles of the body into action. But under normal circumstances 

 the irradiation is limited to certain lines, which increase in number 

 with the strength of the entering impulse. The general pattern of 

 the nervous web remains fairly constant, but its details are subject 

 to great variations, and a new stimulus may act like a tap on a 

 kaleidoscope, and throw a new pattern into being. 



At the commencement of every reflex arc is a receptive neuron 

 extending from a sensory surface to the brain or cord, and this is a 

 private path exclusively occupied by impulses from its own receptive 

 points on the surface of the body. These impulses pass along 

 certain association tracts or internuncial paths in the central nervous 

 system, and at the termination of the arc we have a final neuron 

 which acts as the conducting link between the central nervous 

 system and the muscle or gland which it supplies. This final neuron 

 does not subserve exclusively impulses generated at one receptive 

 source, but can be used in the conduction of impulses generated at 

 many points of the body's surface. The arm muscles, for instance, 

 can be thrown into play in response to visual, auditory, tactile, and 

 other sensations. The final neuron thus differs from the initial 

 neuron in being public, not private, and may be spoken of as the 

 final common path. Of course, in every reflex action we are not 

 really concerned with individual neurons, but with thousands of 

 them acting in harmony ; still, for descriptive purposes, it is well to 

 speak of one set of neurons only as a sample of the rest. An ordinary 

 motor nerve is thus a collection of many final common paths. 



Now let us suppose that two stimuli are acting on different parts 

 of the body's surface, each of which would produce impulses that 

 ultimately reach the same final common path together, though they 

 may throw the motor organ into action in rather a different way. 

 Under such circumstances, it is found that the occupation of the 

 public path by one impulse prevents it being simultaneously used by 

 the other ; one reflex or the other takes place, but not both of them. 



For the investigation of such a problem, the " scratch reflex " of 

 the dog is one that lends itself admirably. This can best be studied 

 in the " spinal " dog, that is in a dog in which cerebral influence is 

 shut off by division of the spinal cord in the lower cervical region. If 

 the skin over a large saddle-shaped area covering the shoulders and 

 back is gently irritated on one side, the hind leg of the same side 

 executes scratching movements, which involve flexor muscles princi- 

 pally ; the rate of scratching is about 4 per second, and each move- 

 ment is presumably a short tetanus. The best "artificial flea" to 



