OF MUSCULAR IRRITABILITY. 439 



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 re- 

 sult of elasticity, or of any simple physical property ; and the contraction, 

 instead of being a result of stimulation through the nerves, is especially ex- 

 cited by changes of temperature in the tissue itself. This endowment, which 

 seems to exist in the greatest amount in certain forms of the non-striated 

 muscle, is called Tonicity. These two modifications of Muscular Contract- 

 ility require a separate consideration. 



2. Of Muscular Irritability. 



575. All Muscular Fibres, which are in possession of vital activity, may be 

 caused to contract by stimuli directly applied to themselves ; and these stimuli 

 may be of different kinds. The simplest is the contact of a solid substance, 

 especially if it be pointed ; thus we may excite contractions in Muscular 

 fibres, by simply touching them with the point of a needle or of a scalpel. 

 Most substances of strong chemical action, such as acids and alkalies, will ex- 

 cite the fibres to contraction, when directly applied to themselves; but the 

 most powerful agent of all is Electricity. If we thus irritate a portion of a 

 muscle composed of striated fibre, the biceps, for example, the fasciculus of 

 fibres which is touched will immediately contract, and that one only ; and the 

 contracted fasciculus will soon relax, without communicating its movements 

 to any other. In fact, the only way to call the entire muscle into contraction 

 at once (since it would be impossible to apply direct irritation to every fasci- 

 culus), is to stimulate it through its nerves. On the other hand, if we apply 

 a similar irritation to a portion of non-striated fibre, as that of the Intestinal 

 canal, the fasciculus which is stimulated will contract less suddenly, but ulti- 

 mately to a greater amount ; its relaxation will be less speedy ; and, before it 

 takes place, other fasciculi in the neighbourhood begin to contract ; their con- 

 traction propagates itself to others ; and so on. In this manner, successive 

 contractions and relaxations may be produced through a considerable part of 

 the canal, by a single prick with a scalpel ; a sort of wave of contraction being 

 transmitted in the direction of its length, and being followed by relaxation. 

 Again, in the Muscular structure of the Bladder and Uterus (which is of the 

 non-striated kind), direct irritation excites immediate and powerful contractions, 

 which extend beyond the fasciculus actually irritated, and produce a great 

 degree of shortening; but they do not alternate in the healthy state with any 

 rapid and decided elongation. In the Heart, which is composed of a mixture 

 of striated and non-striated fibre, the Muscular substance of a large part of the 

 organ is thrown into rapid and energetic contraction, by a stimulus applied at 

 any one point; and this contraction is speedily followed by relaxation, which 

 is again succeeded by a number of alternating contractions and relaxations. 

 And in the muscular tissue of the middle coat of the Arteries, which is of the 

 non-striated character, the contraction takes place rather after the manner of 

 that of the bladder and uterus ; a considerable degree of shortening being 

 effected, by the contraction of other fasciculi than those directly irritated, and 

 this shortening not giving way speedily to relaxation ; but a prolonged appli- 

 cation of the stimulus is often necessary to produce the effect. 



576. On the other hand, when the stimuli which excite Muscular Contrac- 

 tility are applied to the nerves, which supply any muscle composed of striated 

 fibre (the Heart only excepted), they produce a simultaneous contraction in the 

 whole muscle ; the effect of the stimulus being at once exerted upon every part 

 of it. The contraction speedily alternates with relaxation, unless the operation 

 of the stimulus be continued, as when an electric current is propagated with- 

 out intermission along the nerve-trunks, in which case the contraction lasts as 



