a Comparison lelween Animal and Vegelahh Life. 87 



dience 7.fhen excited hy those powers speaks for itself. Bit 

 what 15 the human cause I leave to better heads than my own. 

 The involuntary motion of tlie human muscle seems always to 

 take place with a jerk or by impulses. Tliis is exactly the ca e 

 with the vet^etable, nor can they either move again till the re- 

 laxafi;)ii of the muscle has again taken place. It would be cu- 

 rious to ascertain whether the measure is as exact in both as in 

 the vegetable*. In the plant there are two impulses, a long 

 and shint one; the latter, half the length of the former ; the first 

 from ball to ball, the second from knot to knot. Both these 

 measures are seen in the Mimosa sensitiva ; when the whole 

 leaflet drop-;, the spiral is moved from ball to ball; luit when it 

 is a le:f only, then the spiral moves from knot to knot. When 

 the stamens rub together to tlirow their dust on the pistil in the 

 mosses, then it is a single impulse ; and when a leaf is moved it 

 is the same; and thsmuisture it occasions nvdoes the spiral, and 

 prepares it for a new contraction ; and so delicate and sensible 

 is it to each increasing light or decreasinc^ moisture, that each 

 minute almost brings its variation; certainlv each passing cloud 

 over the sun, its change. A few words, however, I must add on 

 the unknown cause of the muscular motion. Tiie human muscle 

 lying so near the outward skin, (a cuticle which though doubled, 

 nay trebled, is so pierced with pores which must admit much 

 light and moisture,) would seem- to sngaest some cait<e for its 

 motion, of the same nature as the vegetable, did not the peculiar 

 sort of cross action in the muscle-cases seem to suggest a dif- 

 ference ; and vet there is one circumstance which is so exactly 

 contrived respecting the nerves, as it is in the vegetable in tlie 

 Tnu:cles, as to carry some impression witl; it: " The sense of 

 touch is supposed to arise from the nervows extremities of the 

 papillcB being more numerous, or covered with a thicker or a 

 i/iinncr skin in some ]ilaces than in others, and thus bestowitig a 

 grosser or a finer degree of feoling to the diuereat part?. Now 

 this is exactly the case with the muscles : they increase their ir- 

 ritability in proportion to their advancement to the m/tward cu- 

 ticle, and as thev become more sensible of the (ffects of the al^ 

 mospiiere. 



In all trees and. shrubs whose branches live through the win- 

 ter, and are therefore exposed to severe changes, the muscles are 

 concealed, within the ivood : but in other plants (rising eacli year 

 from the earth) they are discovered next the alburnum vessels ; 

 mid in all very sensitive plants the oatward cylinders of the rind 

 are few in nundjcr and very thin, and the more irritable the 

 t^riusclc the thinner they are. Now there is some analogy here, 



* But ill the liuinaii, I believe, it flcpund? on liic Icngtli of die muscle, 



F 4 which 



