578 HANDBOOK OF PHYSIOLOGY. 



these include a much larger number of the natural and purposive move- 

 ments of the lower animals than of the warm-blooded animals including 

 man: and that over nearly all of them the mind may exercise, through 

 the higher nerve-centres, some control; determining, directing, hinder- 

 ing, or modifying them, either by direct action, or by its power over 

 associated muscles. 



To these instances of spinal reflex action, some add yet many more, 

 including nearly all the acts which seem to be performed unconsciously, 

 such as those of walking, running, writing, and the like: for these are 

 really involuntary acts. It is true that at their first performances they 

 are voluntary, that they require education for their perfection, and are 

 at all times so constantly performed in obedience to a mandate of the 

 will, that it is difficult to believe in their essentially involuntary nature. 

 But the will really has only a controlling power over their performance ; 

 it can hasten or stay them, but it has little or nothing to do with fche 

 actual carrying out of the effect. And this is proved by the circum- 

 stance that these acts can be performed during complete mental abstrac- 

 tion : and, more than this, that the endeavor to carry them out entirely 

 by the exercise of the will is not only not beneficial, but positively in- 

 terferes with their harmonious and perfect performance. Any one may 

 convince himself of this fact by trying to take each step as a voluntary 

 act in walking downstairs, or to form each letter or word in writing by 

 a distinct exercise of the will. 



These actions, however, will be again referred to. 



Morbid reflex actions. The relation of the reflex action to the strength 

 of the stimulus is the same as was shown generally to occur in nerve- 

 centres, a slight stimulus producing a slight movement, and a greater, 

 a greater movement, and so on; but in instances in which we must 

 assume that the cord is morbidly more irritable, i.e., apt to issue more 

 nervous force than is proportionate to the stimulus applied to it, a slight 

 impression on a sensory nerve produces extensive reflex movements. 

 This appears to be the condition in the disease called tetanus, in 

 which a slight touch on the skin may throw the whole body into 

 convulsions. 



Special Centres. It may seem to have been implied that the spinal 

 cord as a single nerve-centre, reflects alike from all parts all the impres- 

 sions conducted to it. This, however, is not the case, and it should be 

 regarded as we have indicated, as a collection of nervous centres united 

 in a continuous column. This is well illustrated by the fact that seg- 

 ments of the cord may act as distinct nerve-centres, in which special 

 co-ordinated muscular actions are represented, and excite muscular action 

 in the parts supplied with nerves given off from them; as well as by the 

 analogy of certain cases in which the muscular movements of single 



