36 KNIGHT DUNLAP 



The unit of psycho-physiological activity is a reaction over 

 an arc, which starts from a sensory neuron terminal, or receptor* 

 passes across two or more synapses, ancl terminates in a modifi- 

 cation of the activity of one of the effectors, of which there are 

 three classes, (a) striped muscle, (b) smooth and cardiac muscle, 

 and (d) glands. No consciousness occurs without a complete 

 reaction, although certain reactions apparently produce no con- 

 sciousness. The difference between the psycho-physiological and 

 the purely physiological reactions 4 is an important subject for 

 investigation, but not urgent for the present discussion, in which 

 we are assuming nothing concerning the reactions which can 

 conceivably be altered be any findings with regard to the dif- 

 ference mentioned. 



In actual life there are no simple arcs. Currents sent hi over 

 different afferent routes are collected in the centers and redis- 

 tributed over many efferent routes. It is nevertheless legitimate 

 . to describe the neuro-muscular functions analytically in terms of 

 simple or unitary arcs and reactions. 



Having regard to the termini of the arcs, we can distinguish 

 three lands of reactions: striped-muscular, smooth-muscular, and 

 glandular. Having regard to both starting places and termini, 

 we shall find it important to distinguish between the arcs which 

 connect similar structures, and which accordingly may be called 

 homeodetic, and those which connect dissimilar structures, and 

 which may accordingly be called heterodetic. We have at present 

 no reason to assume that smooth-muscular and glandular reac- 

 tions may not be psychological: but we are certain that the 

 striped-muscular reactions have a large share hi conditioning 



* The "receptors" are: the rod-cells and cone-cells of the retina; the hair- 

 cells of the internal ear; the gustatory cells of the taste-buds; the olfactory cells; 

 the various corpuscles and bulbs in which sensory nerve fibers terminate in the 

 skin, mucous membrane, and connective tissue: the "free" tendings of sensory 

 fibers in various tissues; and the muscle-spindles which lie in the voluntary 

 (striped) muscles and are the specific receptors for the "muscle sense." There 

 Are apparently no sensory endings in glands, and it is questionable whether the 

 afferent terminals in connection with smooth muscle are normally sensory; i.e., 

 whether normally than can initiate conscious reflexes. 



4 By "purely physiological" reactions I mean those reactions which do not 

 directly produce, or condition, consciousness. 



