CHAP, ii.] THE CONTRACTILE TISSUES. 161 



muscles are thrown .into contraction only by nervous impulses 

 reaching them along their nerves; spontaneous movements of 

 the skeletal muscles, that is contractions arising out of changes 

 in the muscles themselves are extremely rare, and when they 

 occur are abnormal; so-called 'cramps' for instance, which are 

 prolonged tetanic contractions of skeletal muscles independent of 

 the will, though their occurrence is largely due to the condition of 

 the muscle itself, generally the result of overwork, are probably 

 actually started by nervous impulses reaching them from without. 

 On the other hand the plain muscles of the viscera, of the intestine, 

 uterus and ureter, for instance, and of the blood vessels very fre- 

 quently fall into contractions and so carry out movements of the 

 organs to which they belong quite independently of the central 

 nervous system. These organs exhibit 'spontaneous' movements 

 quite apart from the will, quite apart from the central nervous 

 system, and under favourable circumstances continue to do this for 

 some time after they have been entirely isolated and removed from 

 the body. So slight indeed is the connection between the move- 

 ments of organs and parts supplied with plain muscular fibres, and 

 the will, that these muscular fibres have sometimes been called in- 

 voluntary muscles; but this name is undesirable since some muscles 

 consisting entirely of plain muscular fibres (e.g. the ciliary muscles 

 by which the eye is accommodated for viewing objects at different 

 distances) are directly under the influence of the will, and, some 

 muscles composed of striated fibres, (e.g. those of the heart) are 

 wholly removed from the influence of the will. 



We shall best study however the facts relating to the move- 

 ments of parts provided with plain muscular fibres when we come 

 to consider the parts themselves. 



Like the skeletal muscles, whose nervous elements have been 

 rendered functionally incapable ( 78), plain muscles are much 

 more sensitive to the making and breaking of a constant current 

 than to induction-shocks ; a current, when very brief, like that of 

 an induction-shock, produces little or no effect. 



The plain muscles seem to be remarkably susceptible to the 

 influences of temperature. When exposed to low temperatures 

 they readily lose the power of contracting ; thus the movements 

 of the intestine are said to cease at a temperature below 19C. 

 Variations in temperature have also very marked effect on the 

 duration and extent of the contractions. Assorted probably 

 with this susceptibility is the rapidity with which plain muscular 

 fibres, even in cold-blooded vertebrates, lose their irritability 

 after removal from the body and severance from their blood- 

 supply. Thus while, as we have seen, the skeletal muscles of a frog 

 can be experimented upon for many hours, (or even for two or three 

 days) after removal from the body, and the skeletal muscles of a 

 mammal for a much less but still considerable time, it is matter of 

 very great difficulty to secure the continuance of movements of the 



F. 11 



