(572 PHYSIOLOGY. 



nections exist between the different parts of the spinal cord. The 

 irritation of the periphery at any point whatever, being transmitted 

 to the spinal cord by a sensitive nerve, goes to provoke at once the 

 activity of the whole organ. 



The initial stimulation for a reflex action may arise from any 

 sensory nerve, whether of special sense, touch, or visceral supply. 

 But there are some which generate a more active reflex movement, 

 among which may be mentioned those of the palm of the hand and 

 the sole of the foot. The quality and nature of the stimulus used 

 has an influence on the reflex. Thus, tickling the auditory meatus 

 produces cough; excessive sunlight acting on the retina cause* 

 sneezing. Stimulation of a sensory nerve-trunk in any part of its 

 course calls out a reflex action, but the movement in this case is much 

 less energetic and its character altered. In such a case the stim- 

 ulation causes movement in one or more muscles, while stimulation 

 of the skin surface innervated by the same nerve produces movements 

 which have a peculiar character of co-ordination. To produce a 

 reflex action the application of the stimulus must be sufficiently 

 rapid. 



Any agent which produces a slow and gradual change in the 

 nerve is without effect. Some experimentalists have found a differ- 

 ence between the reflex of chemical and of mechanical stimulation. 

 When the reflex center has a greater or less excitability, then the 

 stimulation produces greater or less results. Every center which 

 gives origin to a motor nerve may be looked upon as a reflex center. 

 The excitability of the reflex centers is increased when their con- 

 nection with the cerebrum is cut off or when the latter centers are 

 inactive. Hence after decapitation, removal of the brain, section of 

 the oblong medulla, or section of the spinal cord, the centers below 

 the section have greatly increased activity in their reflexes. Set- 

 schenow has shown that mainly in the optic thalami and corpora 

 quadrigemina are seated centers inhibiting the activity of the spinal 

 reflex centers. 



Reflex excitability is much greater in young animals than in 

 adults. This explains the quickness with which slight causes pro- 

 duce convulsions in the infant. Reflex activity is greater in the 

 summer than in the winter. Certain toxic agents have an effect on 

 the reflexes. Thus, atropine, bromides, chloral, chloroform, and 

 ether reduce reflex activity, while strychnine greatly excites it. 

 Chloroform is poisonous to every living cell, whether of plant or of 



