350 DISEASES OF THE PERIPHERAL NERVES. 



a series of morbid impressions in quick succession, the muscular con- 

 traction resulting from the first impression will continue until the second 

 3ne succeed it. In this way permanent muscular contractions occur 

 which are called tonic spasms. If the nerve be irritated at somewhat 

 longer intervals, so that the muscle relaxes ere another impression 

 make it contract again, thus causing alternate spasm and relaxation, we 

 speak of clonic spasms. 



In the healthy subject the motor nerves receive a portion of their 

 stimulus from the grand centre of the will, thus producing the voluntary 

 movements ; another portion of it is obtained from parts of the brain, 

 which, when thrown into excitement, are independent of the action of 

 the will. To this class belong the movements which occur during the 

 painful emotions, and in anger, and which we call outbursts of pain 

 and anger. It is not through our will that we contort the face in 

 weeping, when we are in grief; we do not voluntarily clinch the fists 

 when angry ; these events occur in spite of our will. Finally, in the 

 healthy subject, there is another series of motions which are likewise 

 brought about without the action of the will, and even against the 

 will, by the transmission of a centripetal impression to a centrifugal 

 channel. These are called normal reflex movements ; and a distinction 

 is made between those induced by a regularly recurring stimulus of 

 moderate intensity, such as the respiratory movement which is called 

 forth by the collection of carbonic acid in the blood, and the act of 

 deglutition which follows upon the introduction of a bolus into the 

 oesophagus (these being called automatic movements), and those in- 

 duced by stronger but less frequent impressions, such as sneezing, 

 winking, and the like. 



The terms cramp, spasm, and hypercinesis, are applied to certain 

 morbid conditions in which excitement of the motor nerves is produced 

 by an irritant of unusual and generally speaking of unknown origin, 

 and entirely independently of the volition, or else to a condition in which 

 the action of an ordinary stimulus calls forth an excitement of extraor- 

 dinary extent and violence in the motor tract. 



It is impossible in the present state of science to classify spasms 

 according to the kind of irritant which gives rise to the nervous ex 

 citement, or according to the region in which the irritant operates 

 Spasms of which we are about to treat are the result of a variety of 

 causes, and often of unknown causes ; and although in some instances 

 the affection is confined to the province of a single motor nerve, yel 

 we are not at all certain that it proceeds from direct irritation of the 

 fibres of that nerve. 



