388 HEALTH AND DISEASE 



proceeding from the exercise of the will. The last always originate within 

 the nerve centre, and the impulses they awaken travel from that centre 

 and expend themselves on muscles or glands. It is possible that some 

 animals may have nerves capable of responding to stimuli of which we 

 have no more conception than a man born blind has of light, as they 

 certainly have nerves which recognize variations in the intensity of 

 ordinary stimuli that are imperceptible to us. Thus the presence of water, 

 or the proximity of one of their own species, is recognized by many animals 

 when quite imperceptible to man. A stimulus may be so feeble that it 

 fails to be propagated to the central nerve-cells, and is then said to be 

 insufficient or inoperative; but though our application of the stimulus may 

 not be thus propagated with such intensity as to excite the cell to dis- 

 charge itself, it may awaken its activity if frequently repeated, and cause 

 it to respond, just as a touch which fails to awaken a sleeper, will do so if 

 repeated. This constitutes the summation of a stimulus. The chemical 

 stimuli are represented by the various mineral and vegetable acids, by 

 alkalis, by ethereal and alcoholic liquids. As a rule, any chemical sub- 

 stance that produces sensation when applied to a sensory nerve, will cause 

 contraction of muscle when applied to a motor nerve, but experiment has 

 shown that the motor nerves are more strongly affected than the sensory 

 by alkaline solutions. 



The spinal cord is primarily to be regarded as formed by the union 

 or joining together of many nerve centres, that is to say, of many groups 

 of cells, which commonly act together or in an orderly sequence, producing 

 purposive actions, without any voluntary effort, beyond perhaps supplying 

 the first incentive or stimulus. Even this may be entirely absent, and 

 the animal may have no consciousness of the nerve stimulus or of the 

 muscular actions that follow it. The complete independence of a segment 

 of the spinal cord is well shown in cases where the spinal cord has been 

 crushed in the region of the back by a fall, or a musket-ball, or a sabre- 

 cut. The voluntary movements of the hind limbs are abolished, the 

 animal can neither move them nor feel any injury inflicted upon. them. 

 It is said to be paralysed, yet if the skin of the limb be pinched, or 

 touched with a hot body, it will immediately respond by kicking, or by 

 some other spasmodic movement of the paralysed limb. Similarly, by 

 appropriate stimulation the bladder or the rectum may be made to dis- 

 charge its contents. Such actions or movements are said to be reflex. 

 It can be shown that a stimulus applied to the skin excites a wave 

 which travels up the nerve, enters the cord by the superior root of one 

 or more of the spinal nerves, and reaches one of the nerve -cells in the 

 superior cornu. From this it passes into the inferior cornu and reaches 



