THE REFLEX ARC 251 



to the cephalic region of reflex arcs with receptors in 

 postcephalic regions undergoes change. Apparently it 

 becomes increasingly possible for excitation in such arcs 

 to affect more anterior levels of the body and finally the 

 cephalic region. In other words, the upward paths 

 along the longitudinal axis apparently become more 

 numerous and more readily traversed by impulses. It 

 has been pointed out, for example (p. 99), that in the 

 wormlike invertebrates even the excitation resulting 

 from section of the body is much less effective at levels 

 anterior than at levels posterior to the section, and less 

 extreme excitations often produce no effect anterior but 

 a marked effect posterior to the level of stimulation. 

 The head region in such forms is to a considerable 

 degree autocratic with respect to posterior levels. In 

 the higher vertebrates, however, excitations from any 

 level of the body may reach even the cerebral cortex, 

 not directly, it is true, but only through the interme- 

 diation of various correlation centers. In the higher 

 vertebrates and man, as compared with other forms, 

 the upward paths in the longitudinal axis have appar- 

 ently become more numerous and more readily traversed 

 by impulses. Even in these forms, however, the upward 

 paths in their definitive condition differ in general from the 

 downward paths in that they usually consist of several 

 neurons, while the downward paths consist of a smaller 

 number or of only one. Functionally the definitive 

 upward paths appear to present obstacles to the passage 

 of impulses, while the downward paths are more simple 

 and more open. 



Special attention must be called in this connection 

 to two facts: first, while this difference between upward 



