908 REFLEX ACTIONS. [BOOK in. 



action, varies in character according to the particular part of the 

 body to which the stimulus is applied. The reflex actions 

 developed by stimulation of the internal viscera are different 

 from those excited by stimulation of the skin. We have reason 

 to think that the contraction of or other changes in a skeletal 

 muscle may produce, by reflex action, contractions of other muscles ; 

 and such reflex actions also differ from those started by stimulation 

 of the skin. In reflex actions started by applying a stimulus to 

 the skin the movements vary largely according to the particular 

 area of the skin which is affected. Thus, pinching the folds of 

 skin surrounding the anus of the frog produces different effects 

 from those witnessed when the flank or toe is pinched ; and, 

 speaking generally, the stimulation of a particular spot calls 

 forth particular movements. In the case of the simpler reflex 

 movements, it appears to be a general rule that a movement 

 started by the stimulation of a sensory surface or region on one 

 side of the body, is developed on the same side of the body, and 

 if it spreads to the other side, still remains most intense on 

 the same side ; the movement on the other side moreover is 

 symmetrical with that on the same side. It has been main- 

 tained that 'crossed' or diagonal reflex movements, as where 

 stimulation of one fore-foot leads to movements of the opposite 

 hind-limb, do not occur unless some portion of the bulb be left 

 attached to the spinal cord. Seeing that locomotion in four- 

 footed animals is largely effected by diagonal movements of the 

 limbs, one would rather have expected to find the spinal cord 

 itself provided with mechanisms to assist in carrying them out ; 

 and indeed it is affirmed that in the case of cold-blooded animals 

 and of many young mammals, after division of the spinal cord 

 below the bulb, a gentle stimulation will provoke a diagonal 

 movement, slight pressure on one fore-foot for example giving 

 rise to movements in the opposite hind-leg; a strong stimulus 

 however will produce an ordinary one-sided movement. Again, 

 when in a dog the cord has been divided in the lower thoracic 

 region so that the hind limbs depend on the lumbar cord alone, a 

 rhythmically repeated drawing up and letting down of the hind 

 limbs is witnessed when these are allowed to hang down ; and 

 these movements, which appear to be of a reflex nature excited 

 by the pendant position of the limbs, are often seen to alternate 

 regularly in the two limbs, the right leg being extended while the 

 left leg is being drawn up and vice versa. It may further be 

 observed that if the foot of one pendant limb be pinched while 

 the other limb is passively flexed the flexion of the limb which is 

 pinched is accompanied by an extension of the other limb. In 

 these respects however different animals, as already urged, differ 

 from each other. 



588. From these and similar phenomena we may infer that 

 the nervous network spoken of above is, so to speak, mapped out 



