MUSCLE. 31 



electrical changes occurring during a voluntary contraction. Using 

 the string galvanometer, about 50 electrical variations per second can 

 be observed in the contracting muscle ; and if the electrical variations 

 of a motor nerve during a voluntary or reflex contraction of the muscle 

 which it supplies are similarly recorded, it is found that about 50 

 impulses per second are passing along the nerve. When a skeletal 

 muscle is stimulated 50 times a second it passes into tetanus ; and it may 

 be concluded that voluntary muscular contractions are almost always 

 tetanic in character, and are brought about by the discharge of rapidly 

 repeated impulses from the cells of the central nervous system. 



UNSTRIATED MUSCLE. 



The muscle fibres which occur in the walls of the digestive tract, 

 blood-vessels, and other organs show no transverse striation and are 

 called plain or involuntary muscle fibres. Each fibre is spindle-shaped 

 and has an oval nucleus ; its cell substance frequently shows a delicate 

 longitudinal striation. The fibres are united to one another by a 

 cement substance which can be stained with silver nitrate. 



The changes taking place in smooth muscle, when it contracts, 

 differ in many respects from those occurring in skeletal muscle. In the 

 first place, the duration of the contraction is very prolonged ; the 

 latent period may be from 0*2 to 0*5 second, and the contraction may 

 last for two or three minutes. Secondly, the muscle' is much more 

 easily excited by a constant current than by induction shocks, and 

 frequently fails to give any response to a single induction shock. 

 Owing to the slowness of the process, the origin of the contraction at 

 the kathode when a constant current is made, and at the anode when 

 the current is broken, can be observed with great ease. 



Thirdly, smooth muscle shows a great tendency to contract rhyth- 

 mically, the rhythm being most easily evoked when the muscle is 

 stretched. The eftect of tension is often well seen in the hollow organs 

 whose walls are partly composed of smooth muscle. If such an organ 

 is rapidly distended by the injection of fluid, the sudden tension 

 placed on the muscle fibres in its wall causes them to contract forcibly, 

 the contraction being sometimes continued in a rhythmic manner for 

 a short time. Smooth muscle is also susceptible to chemical stimuli 

 and can be thrown into contraction by substances such as barium salts. 



One of the most characteristic features of smooth muscle is its 

 power to remain in a state of partial contraction or tone, even after 

 it is cut off from any connection with the central nervous system. 

 Stimulation of the nerves to the muscle may bring about either an 

 increase or a decrease of this tone ; and most unstriated muscles have 



