THE TISSUES 91 



neurons, the cerebellar ingoing neurons, the outgoing cere- 

 bellar neurons, either direct to the cord or through the 

 cerebrum, and the peripheral outgoing neurons. 



A. Simple Reactions. 



Reflex Action. This has already been considered shortly 

 in dealing with the stimulation of outgoing neurons through 

 their synapses with ingoing neurons. It has been shown 

 that considerable time is occupied in the passage of the 

 stimulus, and that the resulting action depends upon the 

 condition of the neurons. In the ordinary reflex action the 

 synapses involved are situated in the spinal cord. 



Spinal Reflex Action. The simplest manifestation of 

 reflex action is to be seen in the effect of pinching the toe 

 of a frog in which the brain has been destroyed so that the 

 spinal arcs can act without interference. If the pinch is 

 gentle the foot is simply drawn up. If the pinch is stronger 

 a more extensive movement occurs, and if the leg is held 

 firmly the opposite limb is drawn up. If a piece of paper 

 dipped in acetic acid is laid on the animal's flank, extensive 

 and well co-ordinated movements are made with one or both 

 limbs to remove the irritant. In all cases the act is a definite 

 and co-ordinated one, bringing about an appropriate, but 

 inevitable reaction of the animal on its surroundings. This 

 is due to the passage of an impulse inwards, which sets 

 up changes in the outgoing neurons. With a very gentle 

 stimulus the effect manifests itself at the same level of the 

 cord, but with more powerful stimuli other levels of the cord 

 are acted upon and a more extensive movement results. 

 Generally speaking, stimulating one hind limb causes a 

 movement first in the limb stimulated, then in the fore limb 

 of the same side, then in the opposite fore limb, and lastly in 

 the opposite hind limb. (Practical Physiology, Chap. XII.) 

 In the " spinal dog " a dog with the spinal cord separated 

 from the brain by section in the dorsal region Sherrington 

 finds that different kinds of stimuli produce different kinds 

 of result. 



In reflex action the shortest arc is usually the line of least 

 resistance along which the nerve-change travels, and more 



