CHAP, ii.] THE CONTRACTILE TISSUES. 133 



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 un- 

 striated 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 longi- 

 tudinal coats of the intestine constitute what is called peristaltic 

 action. 



Like the contractions of striated muscle the contractions of 

 plain muscles may be started by stimulation of nerves going 

 to the part, the nerves supplying plain muscular tissue, running 

 for the most part as we have said 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 im- 

 portant distinction between the striated skeletal muscles, arid 

 the plain muscles of the viscera. . As a general rule the skeletal 

 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 

 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 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- 



