Musculab Tissue 39 



THE CONTRACTION OF CARDIAC MUSCLE. 



Cardiac muscle, in its normal condition, like striated muscle in Bieder- 

 mann's solution, contracts periodically without a periodic stimulus. The 

 heart of a living animal receives excitatory currents from nerves of the 

 ' sympathetic ' system, and inhibitory currents from the vagus nerve ; but 

 these do not cause the periodic activity of the heart muscle. In reptiles 

 and many other animals, the heart continues its contractions after being 

 completely isolated from the nervous system, and even after being re- 

 moved from the body. If the excitability of the excised heart be increased 

 by immersing it in Biedermann's solution or one of certain other saline 

 solutions, the beating may be prolonged for hours. 



The stimulus producing the contraction is, in the case of the excised 

 heart, internal ; such can be considered to be the case in the undetached 

 living heart. The nerve currents serve but to increase (or decrease) the 

 excitability of the cardiac muscle, and hence to accelerate (or retard) the 

 spontaneous activity. Chemicals which modify the excitability of the ex- 

 cised heart also modify the excitability of the heart in situ. Cardiac 

 muscle, in addition to its power of periodic contraction without periodic 

 stimulation from outside, and even without any external stimulation, re- 

 sponds to a single external stimulus by a single twitch, as does striated 

 muscle. The contraction of the heart muscle so brought about differs 

 from the corresponding contraction of striated muscle in three particu- 

 lars. ( 1 ) The excitation may pass from fiber to fiber directly, because of 

 the protoplasmic connections between fibers. (2) The degree of contrac- 

 tion is not dependent upon the intensity of the stimulus. If a fiber con- 

 tracts at all, it contracts with the full force of which it is capable. A 

 stimulus which will not produce full contractions will produce no contrac- 

 tions at all. This principle is the "all or nothing" or "all or none" law. 

 (3) Summation of contraction and tetanus do not occur in the case of the 

 cardiac muscle. While the muscle is contracting, a stimulus has no effect 

 on it. After the contraction, it becomes again irritable. The interval dur- 

 ing which the muscle is unexcitable is called the refractory period. 



THE CONTRACTION OF SMOOTH MUSCLE. 



Smooth muscle resembles cardiac muscle in having active power of its 

 own. Cut off from all nervous connection, it still may contract and relax 

 alternately if subjected to a continuous external stimulus, tension, for ex- 

 ample. In general smooth muscle responds to all the artificial stimuli to 

 which striated muscle responds, and it also responds to various drugs, such 

 as digitalis, ergot, salts of barium, etc., which produce different effects on 

 smooth muscle of different organs. 



