Muscular Tissue 37 



ulus, and with the temperature, tonus, (tone; vide infra, Chapter IX), 

 and fatigue of the muscle, and with the work done. The frog's gastroc- 

 nemius muscle, at ordinary room temperature may, when excited by an 

 induction coil current, give such results as: latent period, lOo-; contraction, 

 40<x relaxing, 50<r; the whole process therefore taking place within T X T sec. 

 (1 second is 1000<r). In the living animal, when the muscle has tone, 

 the times may be much shorter. 



The amount of shortening which a striated muscle displays is influ- 

 enced by the same conditions which control the time relations of the pro- 

 cess. Under similar conditions of temperature, fatigue, etc., a weak stimu- 

 lus may cause a slight contraction, a stronger stimulus a more pronounced 

 contraction. There is, of course, a maximum for each muscle. 



The same muscle under otherwise similar conditions, will contract less, 

 if the ' load ' or work done be greater. In the excised frog's muscle, the 

 upper end may be rigidly supported, and a weight attached to the lower 

 end. The extent of contraction will then decrease with increasing ' load '. 

 If both ends be rigidly fastened, we may produce ' contraction ' without 

 shortening ; the muscle being under longitudinal tension during the moment 

 of activity. 



When stimulated by ordinary means, the whole muscle fiber does not 

 contract simultaneously. Contraction begins at the point at which the 

 stimulus (whether nerve current or artificial stimulus) is applied, and 

 spreads in both directions. The rate of propagation of the contraction is 

 3 to 4 meters per second in the frog's muscle, and may be as high as 6 

 meters per second in muscles of warm-blooded animals. The contraction 

 is strictly limited to the fibers stimulated ; and does not directly affect 

 adjacent fibers. The contraction of a fiber may, however, cause contrac- 

 tion of other fibers in contact with branches of the same nerve axon, the 

 stimulation of one branch by the contraction of the muscle fiber in contact 

 with it being transmitted to the other branches and from them to the fibers. 



When successive stimulations are applied to a muscle at such rate (15 

 to 40 per second, according to conditions), that before the contraction 

 produced by one has ceased, another has occurred, the result is a single 

 contraction, maintained as long as the stimulations continue, and much 

 greater in extent than the contraction which would be produced by a single 

 stimulus of the series. Such a state of contraction is called tetanus. If the 

 stimuli are applied at a rate too slow to produce tetanus, but so that the 

 successive stimuli arrive before the effect of the preceding one has com- 

 pletely disappeared (i. e., before the end of the period of relaxing), the 

 result is a series of contractions much greater in extent than the contrac- 

 tion resulting from a single stimulus of the same sort. This condition is 



