in their Structure and Functions, 25 



wrinkled, and its two extremities approximate, so that the 

 parts to which they are attached are drawn nearer together : 

 thus, when the biceps flexor muscle is made to contract, the 

 forearm and hand are raised towards the shoulder. Con- 

 tractility, or, as it is called by some, irritability, is peculiar to 

 muscular fibre, and does not belong to any other animal solid 

 whatever. Blumenbach has, indeed, ascribed to membrane a 

 kind of contractile power ; but the example of it which he 

 adduces can, with more propriety, be referred to elasticity. 

 Muscular contraction never takes place, except on the applica- 

 tion to the muscular fibre of some irritating agent, which is 

 called a stimulus. These agents or stimuli are exceedingly 

 numerous, and consist of mechanical impulses of every degree, 

 of a variety of chemical substances, of electricity, and of 

 volition. The older physiologists endeavoured to account for 

 contractility on mechanical principles, and to refer it to the 

 operation of elasticity ; but, if we carefully note the phenomena 

 that attend it, we shall perceive that it bears no analogy what- 

 ever to this or any other species of mechanical force. 



In the first place, the contraction of a muscle is always 

 greater than the cause by which it is produced : so that it 

 appears that in contractility there is an actual generation of 

 power ; whereas, in the reaction of elastic bodies, the effect 

 produced can never exceed the amount of the cause producing 

 it. In the second place, in contractility, the reaction com- 

 mences during the application of the mechanical agent ; but in 

 elasticity, the reaction cannot take place until the force that 

 had been impressed is removed. For example, if a straight 

 piece of whalebone be bent into a curve, it cannot regain its 

 original form until one of its extremities be liberated ; but, if 

 the surface of a muscle be scratched with a sharp-pointed 

 instrument, its fibres begin to contract the very moment the 

 operation is commenced. In the third place, the contraction 

 of muscular fibres, after continuing some time, is succeeded 

 by a state of relaxation or exhaustion, even although the 

 stimulus continues to be applied to them ; and muscles which 

 have become exhausted or relaxed, after having been in a state 

 of contractility, cannot be again excited to act until a certain 

 period has elapsed. In mechanics, there is nothing which bears 

 the slightest resemblance to this ; for in every description of 

 machinery a certain effect will always be produced so long as 

 the application of an efficient cause is continued. Such are 

 the properties of muscular fibre, and such are the phenomena 

 attendant upon and peculiar to contractility. 



We shall now proceed to examine some of the actions of 

 vegetable organs. If we can discover that these actions are 

 accompanied with phenomena such as we know can only be 



