CULM III. 1. 1. OP VOLITION. 275 



CLASS III. 



DISEASES OF VOLITION. 



ORDO I. 



Increased Volition. 



GENUS I. 

 With increased Actions of the Muscles. 



WE now step forward to consider the diseases of volition, 

 that superior faculty of the sensorium, which gives us the power 

 of reason, and by its facility of action distinguishes mankind 

 from brute animals; which has effected all that is great in the 

 world, and superimposed the works of art on the situations of 

 nature. 



Pain is introduced into the system either by excess or defect 

 of the aciion of the part. (Sect. IV. 5.) Both which circum- 

 stances seem to originate from the accumulation of sensorial 

 power in the affected organ. Thus when the skin is exposed to 

 great cold, the activity of the cutaneous vessels is diminished, and 

 in consequence an accumulation of sensorial power obtains in 

 them, because they are usually excited into incessant motion by 

 the stimulus of heat, as explained in Sect. XII. 5. 2. Contrary- 

 wise, when the vessels of the skin are exposed to great heat, an 

 excess of sensorial power is also produced in them, which is de- 

 rived thither by the increase of stimulus above what is natural. 



This accounts for the relief which is received in all kinds of 

 pain by any violent exertions of our muscles or organs of sense; 

 which may thus be in part ascribed to the exhaustion of the sen- 

 sorial power by such exertions. But this relief is in many cases 

 so instantaneous, that it seems nevertheless probable, that it is 

 also in part owing to the different manner of progression of the 

 two sensorial powers of sensation and volition; one of them com- 

 mencing at some extremity of the sensorium, and being propa- 

 gated towards the central parts of it; and the other commencing 

 in the central parts of the sensorium, and being propagated to- 

 wards the extremities of it; as mentioned in Sect. XI. 2. 1. 



These violent voluntary exertions of our muscles or ideas to 

 relieve the sensation of pain constitute convulsions and madness; 

 and are distinguished from the muscular actions owing to in- 

 creased sensation, as in sneezing, or coughing, or parturition, or 



