190 ANIMAL PHYSIOLOGY. 



nervous centre taking part in reflex actions; for, if tlie 

 paralysed limbs be pinched or otherwise irritated, their 

 muscles are contracted so as to draw them up, as if under 

 the influence of the will, but without the animal evincing 

 any knowledge of either the irritation or the movement; the 

 explanation being that the irritation causes an impression to 

 be transmitted by the sensory nerves to the cord, and this 

 is thence reflected in such manner down the motor nerves 

 as to produce a co-ordinated contraction of the muscles. 



The same points are still better illustrated in those 

 instances in which the spinal cord has been injured by 

 violence in man, so as practically to divide it. If, in such a 

 case, the soles of the feet are tickled, the limbs are drawn 

 up, but the patient is able to tell you that he is quite uncon- 

 scious of the whole matter. And here it is to be noticed 

 that reflex movements are occasioned much more easily in 

 such a case than in healthy circumstances, when the com- 

 munication with the brain is uninjured. It is as if the con- 

 sciousness exercised a control over the tendency to reflex 

 action; or as if the force were carried on to the brain, 

 instead of being reflected back by the motor nerves. So 

 also, reflex action takes place more easily during sleep than 

 when one is awake; and to this is to be attributed the ease 

 with which, during sleep, deep parts are affected by irrita- 

 tions applied to the surface over them, so that exposure of 

 the chest to cold during sleep is more dangerous than at 

 other times. 



When one side of the spinal cord of an animal is divided in 

 the dorsal region, sensation is lost in the hind limb on the 

 opposite side, and movement on the same side as the opera- 

 tion. When a longitudinal division is made down the middle, 

 separating one lateral half of the cord from the other, in the 

 region from which spring the nerves to either fore or hinder 

 limb, sensation is destroyed on both sides, while motion is 

 unaffected. It is plain, therefore, that the tracts through 

 which sensory impressions are conducted cross the middle 

 line soon after the entrance of the posterior roots of nerves 

 into the cord, while motor impressions pass directly down. 

 But immediately above the spinal cord, in the fore part of 

 the portion of the brain termed the medulla oblongata, which 



