x REFLEX ACTION 169 



and partly of a delicate fibre-cellular tissue called neuroglia^ 

 in which the other elements are imbedded. 



Functions of the Nervous System : Reflex Action. In 

 the fourth chapter you learned that a muscle may be 

 made to contract by a stimulus applied either to the muscle 

 itself or to its nerve. You are now in a position to pursue 

 the subject of the control of various parts of the body by 

 the nervous system a little further. 



A frog is either decapitated or pithed, i.e., the medulla 

 oblongata is severed and the brain destroyed (see p. 103) : 

 there can thus be no question either of sensation or of 

 voluntary action on the frog's part. It is then hung up 

 by a hook or string, so that the legs are allowed to hang 

 freely. If one of the toes is pinched with the forceps, 

 the foot will be drawn up as if to avoid the pinch; or, 

 if some very weak acid be applied to a toe, the foot will 

 again be withdrawn, being raised every time it is touched 

 with the acid with the regularity of a machine. Again, if 

 acid be applied to various parts of the body, the foot of the 

 same side will immediately try to rub off the irritating 

 substance ; or if that foot be held down, the other will come 

 into play. 



Movements of this kind are called reflex actions : the 

 stimulus applied to the skin is transmitted by sensory nerve- 

 fibres to the spinal cord, where it is, as it were, reflected 

 in another form, and passed along motor fibres to one or 

 more muscles, causing them to contract (p. 60). 



As already stated, the spinal nerve-trunks are mixed, i.e., 

 contain both sensory and motor fibres. It has been found 

 by numerous experiments that as the nerve approaches 

 the spinal cord these two sets of fibres separate from one 

 another, the sensory passing into the cord by the dorsal 

 root, the motor by the ventral root. As a consequence of 



