REFLEX ACTIONS. 137 



skin is soon absorbed and its effect is shown at first by a greater 

 sensitiveness to cutaneous stimulation, the slightest touch to 

 the foot causing its withdrawal. Soon, however, the response, 

 instead of being orderly and purposeful, becomes convulsive. A 

 mere touch of the skin or a current of air will throw every muscle 

 into contraction, and the extensors being stronger than the flexors 

 the animal's body becomes rigid in extension at every stimulation. 

 The explanation usually given for this result is that the strychnin, 

 acting upon some part of the nerve cells, increases greatly their 

 irritability, so that when a stimulus is sent into the central nervous 

 system along any sensory path from the skin it apparently radiates 

 throughout the cord and acts upon all the motor cells. This latter 

 supposition leads to the interesting conclusion that all the various 

 motor neurons of the cord must be in physiological connection, either 

 direct or indirect, with all the neurons supplying the cutaneous sur- 

 face. The further fact that under normal conditions the effect of a 

 given sensory stimulus is manifested only on a limited and practically 

 constant number of the motor neurons seems to imply, therefore, 

 that normally the paths to these neurons are more direct and the 

 resistance, if we may use a somewhat figurative term, is less than 

 by other possible paths. Muscular spasms are observed under a 

 number of pathological conditions, for instance, in hydrophobia. 

 We are at liberty to assume in such cases that the toxins produced 

 by the disease affect the irritability of the cells in much the same 

 way as the strychnin. 



Theory of Co-ordinated Reflexes. The purposeful character 

 of the co-ordinated reflexes in the frog gives the impression to the 

 observer of a conscious choice of movements on the part of the 

 brainless animal. Most physiologists, however, are content to see 

 in these reactions only an expression of the automatic activity 

 of a mechanism. It is assumed that the sensory impulses from 

 any part of the skin find, on reaching the cord, that the paths to 

 a certain group of motor neurons are more direct and offer less 

 resistance than any others. It is along these paths that the reflex 

 will take place, and we may further assume that these paths of 

 least resistance, as they have been called, are in part preformed 

 and in part are laid down by the repeated experiences of the indi- 

 vidual. That is, in each animal a definite structure may be sup- 

 posed to exist in the cord; each sensory neuron is connected with 

 a group of motor neurons, to some of them more directly than to 

 others, and we may imagine, therefore, a system of reflex apparatuses 

 or mechanisms which when properly stimulated will react always 

 in the same way. And, indeed, in spite of the purposeful character 

 of the reflexes under consideration their automaton-like regularity 

 is an indication that their production is due to a fixed mechanical 



