138 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



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 adapted to a useful end, 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 con- 

 clusion that all the various motor neurons of the cord must be in 

 physiological connection, either direct or indirect, wdth all the 

 neurons supplying the cutaneous surface. 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 that offered 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 adapted character 

 of the reflexes under consideration their automaton-like regularity 

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



