x REFLEX ACTION 169 



the grey from the white matter, losing their medullary 

 sheath as they do so, and partly of a delicate supporting 

 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 : there 

 can be thus 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 will again be withdrawn, being raised every time it is 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, which, how- 

 ever, cannot be distinguished from one another structur- 

 ally. 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 fibres 



