232 HUMAN PHYSIOLOGY 



By stimulation of a sensory spot, uncoordinated contrac- 

 tions of larger groups of muscles, even of all the muscles, 

 may take place. Examples: convulsions during teething; 

 convulsions in strychnine poisoning. In adults convulsive 

 reflexes seldom occur, and only after very strong stimulation 

 (in intense neuralgia). 



As a stimulation of any sensory fibre may call forth con- 

 vulsive reflexes, connections between all the sensory fibres 

 and all the motor fibres must exist. These connections are 

 normally irritable, but not all to the same extent, so that 

 the impulse radiates over certain tracts and thus causes 

 orderly reflexes. 



Reflex time is the time elapsing between the entrance of 

 the impulse into the spinal cord and the passing out through 

 the motor paths. It is determined by measuring the time 

 elapsing between the beginning of the stimulation and the 

 beginning of the contraction, and subtracting from it the 

 latent period of the muscle and the time taken by the stimu- 

 lation to travel through the sensory and motor nerves. The 

 reflex time is 0.008-0.015 second. 



Influences affecting the reflexes. Reflex action depends 

 upon 



(tf) TJic strength of the stimulus. To call forth a reflex 

 the stimulus must be of a certain strength. Very strong 

 stimulations, however, can inhibit the reflexes. The reflex 

 time is, within certain limits, the shorter the stronger the 

 sensory stimulation. 



(ft) The number and sequence of the stimuli. A greater 

 number of successive and weaker currents can more readily 

 call forth reflexes than a single strong induction current. 



(y) The plaec of stimulation. The reflexes are more 

 easily called forth by stimulation of the sensory apparatus 

 in the skin than by direct stimulation of the nerve trunk. 



Reflex irritability is increased by certain poisons (strych- 

 nine) and during tetanus. It is greater in children than in 

 adults. 



It is decreased by certain poisons (chloroform, morphine, 



