LOCALIZATION OF NERVOUS DISEASES. 333 



form rarely happens. Lesions may be localized in various 

 ways, affecting even very limited areas of motor or sensory 

 tracts of the cord, or they may be confined to particular 

 groups of cells ; in which cases motion or sensation, more or 

 less extensive, will be interfered with, or special sets of muscles 

 affected. 



All these lesions may be irritative or destructive, tending, it 

 is believed, respectively to produce excitation or depression of 

 nervous activity. 



Another noticeable fact in connection with diseases of this 

 system is, that to a greater extent, probably, than in any other 

 group of organs, we find very serious derangement of function, 

 without being able to associate this with any structural 

 change. 



CHAPTER III. 



DISORDERS AFFECTING MOTION AND SENSATION. 



From a consideration of the respective influences which dis- 

 turbance of these two great functions of the nervous system 

 have upon the value of the animals affected, the comparative 

 rarity of their separate occurrence, with the usual ascendency 

 of the motorial in the compound affection, together with the 

 fact that objective symptoms are those which in all cases most 

 attract our notice, motorial diseases are, to us, always of the 

 greatest importance. 



I. Spasms and Convulsions. 



Spasms are involuntary muscular contractions, or move- 

 ments occurring independent of the will. In these the 

 nervous force is transmitted in irregular fits of greater or less 

 continuance. Attended with painful sensations they are 

 known as cramp, when the paroxysms are of short duration, 

 succeeding each other rapidly ; but alternated with periods of 

 repose, they are spoken of as clonic sjxisms. When of con- 

 siderable duration, or terminating in persistent rigidity, they 

 are known as tonic sj^asms. These tAvo forms are seen in the 

 horse respectively in common cranap or in string-halt, and in 

 tetanus. 



