132 SPECIAL PHYSIOLOGY. 



of the muscular substance ; for experiments show that this is very 

 slight, if it occurs at all. Thus, the arm of a man has been inclosed 

 in a glass cylinder having a narrow upright glass tube connected with 

 it, the part of the cylinder not occupied by the arm being filled with 

 water (Glisson) ; again, the prepared hind-limb or limbs of frogs, have 

 been suspended in a bottle of water, provided with an upright capillary 

 tube connected with its neck (Sharpey, Weber, Valentin, and others). 

 The muscles of the arm have been then brought into play, or, the 

 muscles of the frog's limbs have been excited to contract by electrical 

 currents passed into them along wires properly fixed for that purpose ; 

 and any change in their bulk, indicated by a fall in the level of the 

 water in the upright tubes, has been noted. By most observers (Pre- 

 vost, Dumas, Matteuci, Sharpey), it is stated that no diminution of 

 bulk occurs under such circumstances, but according to Ermann, 

 Weber, and Valentin, a diminution, scarcely perceptible in careful 

 experiments, amounting only to from 73^3$ to 73^33 of the bulk of 

 the contracting muscles, takes place. 



When a muscular fibre relaxes it resumes its previous length, and 

 at the same time diminishes in thickness ; and if its points of attach- 

 ment remain stationary, or at the same distance from each other, the 

 fibre is thrown necessarily into zigzag flexions, specially noticed by 

 Prevost and Dumas, and at first erroneously supposed to be produced 

 in the active state of the muscle, and to account for its shortenings. 

 During contraction the sarcolemma of each fibre is passive, and is 

 either thrown into minute folds, or else displays a feeble elasticity. 

 The contractile property, indeed, resides entirely in the sarcode, or 

 peculiar fibrillar contents of the tube of the sarcolemma. 



According to Helmholz, the contraction of a muscle is not instanta- 

 neous, but a certain interval of time, about T J(jth to ^th of a second, 

 elapses between its stimulation by electricity, and its actual contrac- 

 tion ; this he names the period of latent contraction or excitation. The 

 muscle at first contracts quickly, then more slowly ; and it takes a 

 longer time when powerful contractions are excited. The velocity of 

 the wave of contraction in the frog's muscle is about forty inches per 

 second (Aby). The rate of motion along any particular muscular 

 fibre is such, therefore, that its contraction may be regarded as almost 

 simultaneous from one end to the other. But there is reason to sup- 

 pose that in any given muscle, certain fibres are undergoing contraction 

 whilst others are at rest, an alternation of labor which would enable 

 a muscle to maintain a longer effort with less exhaustion or fatigue. 



The amount of contraction which occurs in a detached frog's muscle 

 is, according to Weber, from 50 to 60, or even 80 per cent, of its 

 length ; that is to say, the muscle shortens to one-half, or even to one- 

 fifth, of its length. In the living animal and man, owing to the re- 

 sistance of antagonistic muscles, and to the structure of the joints, the 

 muscles shorten themselves only about one-third of their length. 



In the frog's muscle the maximum amount of shortening takes place 

 when the contraction is sudden, when the muscles are not fatigued by 

 previous stimulation, and when the resistance offered by weights ap- 

 pended to them is slight. 



