174 LOCOMOTION. [CHAP. vn. 



consists of the resistance of their contents, it sometimes happens 

 that these, when removed, are not at once replaced ; and hence an 

 enduring contraction, though the active contractile force is no longer 

 exerted. Thus an empty intestine is reduced to the size of a to- 

 bacco-pipe, and the sphincters of the anus and bladder are kept 

 contracted, without any tetanic spasm, or permanent expenditure 

 of contractile force, as has been sometimes supposed. 



Now, the stimulus of distension is, in the first instance, nothing 

 more than the elongating force which calls into play the contrac- 

 tility of a muscle under its passive form ; and there is this peculiar 

 to it, that it affects equally every point of the substance of each 

 fibre, which no other stimulus can do : and hence would result 

 the uniformity which will presently be shewn to characterize pas- 

 sive contraction, for contraction is an answer to a stimulus. This 

 consideration tends strongly to confirm the view which we have 

 taken of the identity of the forces displayed in passive and active 

 contraction, of tonicity and contractility. 



Other stimuli may be mentioned as capable of causing muscular 

 contraction by their direct agency on the tissue ; but it is important 

 to observe that these take no share in the production of natural 

 contraction in the healthy body. It was long supposed impossible 

 to observe the effect of stimuli on the muscular tissue when isolated 

 from the nervous; and the fact, that the artificial stimuli which 

 induced contraction when applied to a muscle itself, were the same 

 with those which induced it when applied to the motor nerves, was 

 considered sufficient proof that in the former case the effect was 

 produced through the medium of nervous tissue still mingled with 

 the muscular. 



But this question has been brought to an issue by the positive 

 observation that fragments of the fibre of voluntary muscle, entirely 

 isolated from every extraneous tissue, whether nerve or vessel, m 

 be made to contract in obedience to a stimulus topically applied 

 them. When such fragments are examined, they are found to con- 

 tract first of all where they have sustained mechanical injury, viz., 

 at their broken extremities ; and, if water be brought into conl 

 with them, it is absorbed, and thereby excites them to contractions, 

 which commence at their surface.* The same thing is frequently 



* Water has long been known as a rapid exhauster of the contractility 

 muscles. " Rigidity is produced almost instantaneously if warm water be i] 

 jected into the arteries of a muscle. The flesh, under these circumstances- 

 becomes pale, increased in bulk, and suddenly hardens. The operation of 



