

CONTRACTILITY OF ORGANIZED TISSUES. 205 



the distinguishing characteristic of Muscular tissue ; that of contract- 

 ing on the application of a stimulus. Some approaches to this property 

 are manifested by certain Vegetable structures. Thus, if the small 

 enlargement at the base of the footstalk of the leaf of the Sensitive 

 Plant be touched ever so slightly, the leaf will be immediately drawn 

 down by the contraction of the tissue of the part irritated. If the leaf 

 itself be touched, the same effect results, but apparently through a diffe- 

 rent channel ; the tissue of the leaf contracts where it is touched, and 

 forces some of its fluid along the vessels of the footstalk into the upper 

 side of the little excrescence at its base, by the distension of which the 

 leaf is forced down. In the Dionoea muscipula, or Venus's Fly-trap, 

 there is a similar transmission of the effect of the stimulus from one 

 part to another ; for the two lobes of the leaf, which form the trap, are 

 made to close together, when an insect settles upon either one of three 

 spines which project from the surface of each lobe, or when the points 

 of these spines are touched with any hard body. Many other instances 

 of Vegetable movement might be brought together. Some of them are 

 obviously produced by an enlargement or contraction of the cells, occa- 

 sioned by variations in the amount of fluid they contain ; and these 

 variations depend upon the hygrometric state of the atmosphere. With 

 these we have nothing to do. But there are many, in which (as in 

 the case of the Sensitive-plant first mentioned) a stimulus applied to a 

 part occasions the immediate contraction of its cells, and a consequent 

 motion in the same part. And there are also several, in which the con- 

 traction produces motion in a distant part, as in the Dionoea ; but this 

 propagation appears to be of a simply mechanical character ; being 

 accomplished through the medium of fluid, which is forced from one 

 part by its own contraction, and caused to distend another. 



346. From these examples, however, it is evident that the property 

 of contractility is not entirely restricted to the Animal kingdom ; and 

 we shall find that the simplest form under which it manifests itself in 

 the Animal body, bears a close relationship with that which is displayed 

 in Plants. The non-striated fibre of the alimentary canal, which is sub- 

 servient to the functions of Vegetative life alone, is called into action 

 much more readily by a stimulus directly applied to itself, than it is in 

 any other mode. Such is not the case, however, with the striated fibre, 

 of which the muscles of Animal life are composed ; this being much 

 more readily called into action by a peculiar stimulus conveyed through 

 the nerves supplying those muscles, than by any other more directly 

 applied to them. 



347. The Contractility of Muscular Fibre shows itself under two 

 forms. Its most obvious and striking manifestations are those that 

 occur in the voluntary muscles and in the heart ; which, when in action, 

 exhibit powerful contractions alternating with relaxations. The property 

 which is concerned in these is distinguished as Irritability. On the 

 other hand, we find that these same muscles exhibit a tendency to a 

 moderate and permanent contraction, which is not shown by them when 

 they are dead, and which cannot therefore be the result of elasticity or 

 of any simple physical property ; this endowment, which seems to exist 



