508 THE NERVOUS SYSTEM. 



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 

 the actual carrying out of the effect. And this is proved 

 by the circumstance that these acts can be performed with 

 complete mental abstraction : and, more than this, that the 

 endeavour to carry them out entirely by the exercise of the 

 will is not only not beneficial, but positively interferes with 

 their harnfonious and perfect performance.. Anyone may 

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

 a voluntary act in walking down stairs, or to form each 

 letter or word in writing by a distinct exercise of the will. 



These actions, however, will be again referred to, when 

 treating of their possible connection with the functions of 

 the so-called sensory ganglia. 



The phenomena of spinal reflex actions in man are much 

 more striking and unmixed in cases of disease. In some of 

 these, the effect of a morbid irritation, or a morbid irri- 

 tability of the cord is very simple ; as when the local 

 irritation of sensitive fibres, being propagated to the 

 spinal cord, exciter merely local spasms, spasms, namely, 

 of those muscles, the motor fibres of which arise from the 

 same part of the spinal cord as the sensitive fibres that are 

 irritated. Of such a case we have instances in the invo- 

 luntary spasmodic contraction of muscles in the immediate 

 neighbourhood of inflamed joints ; and numerous other 

 examples of a like kind might be quoted. 



In other 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 sensitive nerve produces ex- 

 tensive reflex movements. This appears to be the condition 

 in tetanus, in which a slight touch on the skin may throw 



