Nervous Tissue 



47 



As to the nature of the nerve current, i. e., what passes when the neuron 

 is irritated at one end, and shortly thereafter at its other end irritates an- 

 other neuron or a gland or muscle cell, we can merely guess. Probably it 

 is a chemical process, analogous to the action in a train of powder, which, 

 when lighted at one end, carries the combustion process to the other end. 



It is customary to classify the neurons as (1) sensory, (2) motor, and 

 (3) associative, according as they (1) conduct towards the brain or spinal 



,fM- 



Fig. 33. Diagram of the simplest possible reflex arc from epidermis through cere- 

 bral cortex to striped muscle. (Cunningham, after Ramon y Cajal.) The arc, as 

 drawn, involves four neurons. According to some, there is another synapse in the 

 midbrain between E and the cortex. There may be intermediate neurons in the cortex, 

 forming links between the afferent neuron E and the efferent neuron A. 



cord, (2) conduct away from the brain or cord (towards muscle or gland 

 cells), (3) conduct from point to point within the brain or cord or be- 

 tween the brain and cord. Better terms for the three classes of neurons 

 are (1) ' centripetal ' or afferent, (2) ' centrifugal ' or efferent, and (3) 

 ' intermediate ' or central. The peculiarity of the various parts of the 

 brain and spinal cord, which are divided more or less completely into 

 laterally symmetrical halves, makes it necessary to further divide the "inter- 

 mediate" neurons of these structures into two classes: (a) those which run 

 across from one half of brain or cord to the other half, (b) those lying 



