118 THE CONTRACTILE TISSUES. 



fibre to fibre, both in the direction of the fibres, as when the whole circum- 

 ference of the intestine is engaged in the contraction, or when the wave 

 travels longitudinally along the longitudinal coat, and also in a direction at 

 right angles to the axes of the fibres, as when the contraction-wave travels 

 lengthways along the circular coat of the intestine, or when it passes across 

 a breadth of the longitudinal coat ; that is to say, the changes leading to 

 contraction are communicated not only in a direct manner across the cement 

 substance uniting the fibres of a bundle, but also in an indirect manner, 

 probably by means of nerve fibres from bundle to bundle across the connective 

 tissue between them. Moreover, it is obvious that even the contraction-wave 

 which passes along a single unstriated fibre differs from that passing along a 

 striated fibre, in the very great length both of its latent period and of the 

 duration of its contraction. Hence, much more even than in the case of a 

 striated muscle, the whole of each fibre must be occupied by the contraction- 

 wave, and, indeed, be in nearly the same phase of the contraction at the 

 same time. 



Waves of contraction thus passing along the circular and longitudinal 

 coats of the intestine constitute what is called peristaltic action. 



Like the contractions of striated muscle the contractions of plain mus- 

 cles may be started by stimulation of nerves going to the part, the nerves 

 supplying plain muscular tissue running for the most part in the so-called 

 sympathetic system, but being, as we shall see, ultimately connected with 

 the spinal cord or brain. Here, however, we come upon an important dis- 

 tinction between the striated skeletal muscles and the plain muscles of the 

 viscera. As a general rule, the skeletal muscles are thrown into contrac- 

 tion only by nervous impulses reaching them along their nerves ; spontane- 

 ous 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 occur- 

 rence is largely due to the condition of the muscle itself, generally the re- 

 sult 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 bloodvessels very 

 frequently 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 favorable circum- 

 stances 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 movements of organs and parts supplied with plain muscular fibres and 

 the will, that these muscular fibres have sometimes been called involuntary 

 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 accom- 

 modated 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 movements 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 ( 76), 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. 



