KEFLEX TIME AND INHIBITION OF EEFLEXES. 639 



sciously or even when our psychical activities are concentrated upon some other object really 

 belong to the category of co-ordinated reflexes. Many complicated motor acts must first be 

 learned e.g., dancing, skating, riding, walking before unconscious harmonious co-ordinated 

 reflexes can again be discharged. The co-ordinated reflex movements of coughing, sneezing, 

 and vomiting depend upon the spinal cord, together with the medulla oblongata. 



The following facts are also important : 



1. Reflexes are more easily and more completely discharged, when the specific 

 end-organ of the afferent nerve is stimulated, than when the trunk of the nerve is 

 stimulated in its course (Marshall Hall, 1837). [Thus, by gently tickling the 

 skin, it is easy to discharge a reflex act, while it requires a strong stimulus to be 

 applied to an exposed sensory nerve in order to do so.] 



2. A stronger stimulus is required to discharge a reflex movement than for the 

 direct stimulation of motor nerves. 



3. A movement produced reflexly is of shorter duration than the corresponding- 

 movement executed voluntarily. Further, the occurrence of the movement after 

 the moment of stimulation is distinctly delayed. In the frog, a period nearly 

 twelve times as long elapses before the occurrence of the contraction, than is 

 occupied in the transmission of the impulse in the sensory and motor nerves 

 (Ilelmholtz, 1854). Thus, the spinal cord offers resistance to the transmission of 

 impulses through it. 



The term "reflex time" is applied to the time necessary for transferring the impulse from 

 the afferent fibre to the nerve-cells of the cord, and from them to the efferent fibre. In the 

 frog it is equal to 0*008 to 0*015 second. The time, however, is increased by almost one-third, 

 if the impulse pass to the other side of the cord, or if it pass along the cord, e. g. , from the 

 sensory nerves of the anterior extremity to the motor roots of the posterior limb. Heat dimi- 

 nishes the reflex time and increases the reflex excitability. Lowering the temperature (winter 

 frogs), as well as the reflex-exciting poisons already mentioned, lengthens the reflex time, whilst 

 the reflex excitability is simultaneously increased. Conversely, the reflex time diminishes as 

 the strength of the stimulus increases, and it may even become of minimal duration (J. 

 Rosenthal). The reflex time is determined by ascertaining the moment at which the sensory 

 nerve is stimulated, and the subsequent contraction occurs. Deduct from this the time of 

 latent stimulation ( 298, I. ), and the time necessary for the conduction of the impulse ( 298) 

 in the afferent and efferent nerves (v. Helmholtz, J. Rosenthal, Exner, Wundt). 



[Influence of Poisons. The latent period and reflex time are influenced by a large number 

 of conditions. In a research as yet unpublished, W. Stirling finds that the latent period may 

 remain nearly constant in a pithed frog for nearly two days, when tested by Tiirck s method. 

 Sodic chloride does not influence the time, nor does sodic bromide or iodide. Potassic chloride, 

 however, lengthens it enormously, or even abolishes reflex action after a very short time, and 

 so do potassic bromide, ammonium chloride and bromide, chloral and croton-chloral. The 

 lithia salts also lengthen the reflex time, or abolish the reflex act after a time.] 



361. INHIBITION OF THE REFLEXES. Within the body there are 

 mechanisms which can suppress or inhibit the discharge of reflexes, and they may 

 therefore be termed mechanisms inhibiting the reflexes. These are : 



1. Voluntary Inhibition. Reflexes may be inhibited voluntarily, both in the 

 region of the spinal cord and brain. Examples : Keeping the eyelids open when 

 the eyeball is touched ; arrest of movement when the skin is tickled. We must 

 observe, however, that the suppression of reflexes is possible only up to a certain 

 point. If the stimulus be strong, and repeated with sufficient frequency, the reflex 

 impulse ultimately overcomes the voluntary effort. It is impossible to suppress 

 those reflex movements which cannot at any time be performed voluntarily. Thus, 

 erection, ejaculation, parturition, and the movements of the iris, are neither direct 

 voluntary acts, nor can they, when they are excited reflexly, be suppressed by the 

 will. 



2. Setschenow's inhibitory centre is another cerebral apparatus, which in the 

 frog is placed in the optic lobes. If the optic lobes be separated from the rest of 

 the brain and spinal cord, by a section made below it, the reflex excitability is 

 increased. If the lower divided surface of the optic lobes be stimulated with a 



