216 VETERINARY PHYSIOLOGY 



attached to a lever with a shght load upon it, the strip of 

 tissue will manifest a beat at a rate of about 12 per minute 

 (Practical Physiology). 



4. Stimulation. — Any sudden change will cause these 

 muscles to contract. 



(1) Mechanical. — A blow or a pinch to the gut will set up a 

 contraction, as is seen when the gut is pinched in strangu- 

 lated hernia. Any sudden stretching may cause a contraction. 



(2) Thermal. — A lowering of temperature causes a con- 

 traction, and the more sudden the change the more powerful 

 is the stimulus. Exposure to a continued low temperature 

 sets up a sustained contraction, as may be seen in the gut 

 after death. By warming the gut this contraction may be 

 made to pass off. 



A sudden increase of temperature also stimulates, and 

 this is sometimes taken advantage of in obstetrical practice 

 by injecting hot solution to make the uterus contract. 



(8) Electrical. — The make and break of a galvanic 

 current will cause a contraction as it does in skeletal muscle 

 (p. 219), and it is more effective than the more sudden 

 variation of a faradic current. 



(4) Chemical. — Some chemical substances cause con- 

 traction, e.g. salts of barium, while some cause relaxation, e.g. 

 nitrites. 



If any stimulus is sufficient to make visceral muscle 

 contract, it causes a full contraction because all the fibres are 

 stimulated at once. Further, the continued repetition of a 

 stimulus, insufficient to cause a contraction, may produce one, 

 just as such repetition may liberate a reflex action fp. 84). 



5. Characters of Contraction. — The contraction which is 

 produced is a slow one. A distinct interval occurs between 

 the application of the stimulus and the contraction — the latent 

 'period. This may occupy nearly a second. A slow contrac- 

 tion and a slow relaxation follow. These may be recorded 

 by the methods used for skeletal muscle (p. 222). 



6. Refractory Period. — If a second stimulus is given 

 before the contraction has passed off, probably no second con- 

 traction will be produced. But, if the relaxation phase is in 

 progress, a second contraction may be superimposed upon the 



