48 



PSYCHOBIOLOGY 



entirely in one half, or in one half and peripheral structures. The (a) 

 class are called commissural neurons, and the name association neurons 

 refers strictly to the (b) class. 



Attempts have been made to extend the " all or none " law (page 39), 

 which is valid for cardiac muscle fibers, and apply it to neuron activity. 

 This extension must be considered as not sufficiently supported at present. 



It is the predominant opinion that the neurons are distinct individuals 

 or structural units, and that where several neurons form a functional series 

 or chain, the axon of one cell is merely in contact with the next cell, or with 

 its dendrite. These points of contact between neurons (the points, that is, 



Mm 



W^kL 



wart- 



Fig. 34. Diagrams of reflex mechanisms in the spinal cord. (Barker, Nervous 

 System, after Kolliker.) Left : two-neuron arc, s, sg, sa, sc, afferent neuron with 

 ganglion cell (sg), and ascending and descending branches and collaterals within the 

 cord, m, m, efferent neurons, with cell bodies in cord. Right : three-neuron arc. Only 

 one collateral (sc), of the afferent neuron is shown, c, c, associative neuron. 



at which the stimulus is passed on from one to the other), are called 

 synapses (singular either synapse or synapsis), or synaptic points 



[Figs. 32-34]. There are however some physiologists of eminence who 

 hold that the neurons anastomose at the synapses; or, at least, that the 

 fibrils {vide infra, page 50) are continuous across the synapse from one 

 neuron to another. 



Both axons and dendrites may have many branches, like the rootlets of 

 a tree. Thus, in certain parts of the nervous system, the terminations of 



