278 THE NERVOUS SYSTEM 



pre-existing state of tonus of the blood-vessels. Respiratory 

 movements are inhibited during the act of swallowing. In 

 all these cases the conversion of a positive into a negative 

 effect must be ascribed to the synapses. 



Another degree in this transformation of a positive into 

 a negative effect is seen in the scratch reflex. The nervous 

 impulse resulting from the stimulus, which may be a 

 constant stimulus, causes at one moment contraction of 

 flexors and relaxation of extensors. At the next moment 

 this effect is transformed into relaxation of flexors and 

 contraction of extensors. , This repeated gives the rhythm 

 of the scratch. Such a reflex is termed by Sherrington 

 a reflex of successive double sign. 



The Final Common Path. — A given group of muscles can 

 be brought into action by the stimulation of any of a 

 large number of receptors. The flexion reflex is induced 

 from any point on a large surface of the hind Umb, the 

 scratch reflex from a large surface on the back. There is, 

 then, centrally a convergence of paths upon every group 

 of motor nerves. The motor neurone upon which so many 

 neurones impinge is therefore called the final common 

 path. 



Spread of Reflex. — When the flexion reflex is induced 

 with stimuh of increasing intensity the movement involves 

 more and more of the musculature of the hind limb. With 

 very weak stimuh only the foot is involved, with strong 

 stimuli the whole limb and even the other parts of the 

 body. In addition, therefore, to there being centrally a 

 convergence of paths there is also a divergence — a radiation 

 from a central focus on the motor side. The spread of 

 the reflex effect from the focus can only be explained by 

 assuming that each afferent fibre comes into connection, 

 directly or indirectly, with several motor cells, and that 

 the synapses between the afferent fibres and the several 

 anterior horn cells present to the afferent impulse varying 

 resistances. Some of these resistances are forced easily, 

 others only with difiiculty. 



