190 ELEMENTARY PHYSIOLOGY 



any stimulus be applied to the spinal portions of the nerves which supply the 

 arm (supposing those nerves to have been severed), yet the sensations will be 

 felt in the fingers and in the skin of the arm, even though communication with 

 the brain the seat of all sensation, is cut off. And, even in the case of an 

 amputated limb, the sensation will still be referred to the lost part by the 

 brain. This will enable us to explain the cause of the sensation called ' pins 

 and needles ' produced by pressure on a nerve at the elbow. The irritation is 

 transmitted direct to the brain by one or more sensory nerves, and the brain 

 simply refers the sensation to the extremities of the irritated nerve. 



We may now study the functions of the spinal cord. When 

 the spinal cord is cut through or injured at any point, all power of 

 voluntary movement and all sensation is lost to every part of the 

 body supplied with fibres from nerves which originate below that 

 point. Therefore the cord is a medium by which motor and 

 sensory impressions are conducted to and from the brain. If we 

 now pinch or otherwise irritate the limbs which are thus cut oft 

 from the brain, they are suddenly drawn up by the contraction of 

 their muscles, and this entirely without any exercise of the will. 

 Such an action is termed a reflex action. It consists of a disturb- 

 ance of sensory fibres which is conducted by a sensory nerve to 

 a nerve centre, and a reflection of this disturbance from the nerve 

 centre through one or more motor nerves, causing a contraction 

 of the muscles in which the motor fibres terminate. It will thus 

 be seen that the spinal cord is not merely a conductor of impres- 

 sions, but that it is a centre for reflex actions. This power is 

 possessed by the grey matter of the cord only, the white substance 

 being simply a conductor of impulses. 



Hundreds of reflex actions are going on continually in our 

 bodies without our knowledge, and among them we include the 

 so-called vital functions of the important organs, the cessation of 

 which would cause instant death. Thus, the action of the res- 

 piratory muscles is the ultimate result of a reflex action. In this 

 case the irritation of the sensory nerves concerned is due to the 

 imperfectly aerated condition of the blood. The impression thus 

 made is conveyed to the medulla oblongata, and is thence reflected 

 through motor nerves which govern the intercostal muscles, the 

 diaphragm, &c. Even walking, reading, and other familiar actions 

 of voluntary muscles may be reflex movements. Of course such 

 movements are sometimes purely voluntary, especially in child- 

 hood, but in time the muscles become so used to certain succes- 

 sions of movements, that they act in their accustomed manner 

 without the control of the will. We may walk without knowing 

 that we are walking ; and, in reading, we often unconsciously 

 utter certain sounds on seeing the signs which have so frequently 

 caused us to produce these sounds voluntarily. 



