*6 MOTIONS OF THE RETINA. SECT. III. i. 



which thwart our paffage. I. To demonftrate that 

 the retina and other immediate organs of fenfe pof- 

 fefs a power of motion, and that thefe motions con- 

 flitute our ideas, according to the fifth and feventh 

 of the preceding affertions, claims our firft attenti- 

 on. 



Animal motions are diftinguiflied from the com- 

 ttiunicated motions, mentioned in the firft feclion, 

 as they have no mechanical proportion to their caufe; 

 for the goad of a fpur on the Ikin of a horfe fhall 

 induce him to move a load of ha^. They differ 

 from the gravitating motions there mentioned, as 

 they aj*e exerted with equal facility in all directions, 

 and they differ from the chemical clafs of motions, 

 becaufe no apparent decompofitions or new combi- 

 nations are produced in the moving materials. 



Hence, when we fay, animal motion is excited 

 by irritation, we do not mean that the motion bears 

 any proportion to the mechanical impulfe of the 

 ilimulus ; nor that it is affected by the general gra- 

 vitation of the two bodies ; nor by their chemical 

 properties, but folely that certain animal fibres are 

 excited into action by fomething external to the * 

 moving organ. 



In this fenfe the ftimulus of the blood produces 

 the contractions of the heart ; and the fubftances 

 we take into our ftomach and bowels irritate them 

 to perform their neceffary functions. The rays of 

 light excite the retina into animal motion by their 

 ftimuius ; at the lame time that thofc rays of light 

 themfeives are phyfically converged to a focus by 

 the inactive humours of the eye. The vibrations of 

 the air ftirnulate the auditory nerve into animal ac- 

 tion ; while it is probable that the tympanum of 

 the ear at the fame time undergoes a mechanical vi- 

 bration. 



To 



