X REFLEX ACTION 169 



and partly of a delicate fibro-cellular tissue called neuroglia, 

 in which the other elements are imbedded. 



Tunctions 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 first chloroformed, and then either decapitated 

 or pithed, i.e., the medulla oblongata is severed and the brain 

 destroyed (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 the foot will again be withdrawn, being raised every time 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 



