NEURONS AND NEURON-CHAINS 55 



spinal cord, or (3) to other levels, in reflex arcs of three or more neurons each; 

 or (4) it may ascend to the brain along an ascending branch of a dorsal root 

 fiber. Here it may travel over one or more of a number of paths, each con- 

 sisting of several neurons, and be finally returned to the spinal cord and make 

 its exit by way of a primary motor neuron. The figure illustrates but a few of 

 the possible paths, many of which we shall have occasion to consider in the 

 subsequent chapters. 



For an incoming impulse a variety of paths are open, one or more of which 

 may be taken according to the momentary resistance of each. There is reason 

 to believe that the resistance to conduction across a synapse may vary from 

 moment to moment, according to the physiologic state of the neurons involved. 

 It is therefore not necessary that every impulse entering by a given fiber shall 

 travel the same path within the central nervous system nor produce the same 

 result. The pathways themselves are, however, more or less fixed, and depend 

 upon the structural relations established among the neurons. Many of these 

 synaptic connections are formed before birth, follow an hereditary pattern, 

 and are approximately the same for each individual of the species. In the child 

 these are illustrated by the nervous mechanisms involved in breathing and 

 swallowing, which are perfect at birth. The newly hatched chick is able to run 

 about and pick up food, acts which are dependent on nervous connections al- 

 ready established according to hereditary pattern. In man and to a less extent 

 in other mammals the nervous system continues to develop long after birth. 

 This postnatal development is influenced by the experience of the individual 

 and is more or less individual in pattern. It is probable "that in certain parts 

 of the nervous mechanism new connections can always be established through 

 education" (Edinger, 1911). 



The neurons which make up the nervous system of an adult man are there- 

 fore arranged in a system the larger outlines of which follow an hereditary pat- 

 tern, but many of the details of which have been shaped by the experiences of 

 the individual. 



