MOVEMENTS OF THE ALIMENTARY CANAL, ETC. 309 



giving subrnaximal as well as maximal contractions ; with increased strength 

 of stimulation the amount of the shortening increases until a maximum is 

 reached. This fact may be observed not only upon isolated strips of muscle 

 from the stomach, but may be seen also in the different degrees of contraction 

 exhibited by the intestinal musculature as a whole when acted upon by various 

 stimuli. 



In his researches upon the movements of the ureter Engelmann l showed 

 that a stimulus applied to the organ at any point caused a contraction which 

 starting from the point stimulated might spread for some distance in either 

 direction. Engelmann interprets this to mean that the contraction wave in 

 the case of the ureter is propagated directly from cell to cell, and this possi- 

 bility is supported by the fact, before referred to, that there is direct proto- 

 plasmic continuity between adjoining cells. This passage of a contraction wave 

 from cell to cell has, in fact, often been quoted as a peculiarity of plain 

 muscle-tissue. In the case of the ureter the fact seems to be established, but 

 in the intestines, where there is a rich intrinsic supply of nerve-ganglia, it 

 is not possible to demonstrate clearly that the same property is exhibited. 

 The wave of contraction in the intestine following artificial stimulation is, 

 according to most observers, usually localized at the point stimulated or is 

 propagated in only one direction, and these facts are difficult to reconcile 

 with the hypothesis that each cell may transmit its condition of activity 

 directly to neighboring cells. Upon the plain muscle of the ureter Engel- 

 mann was able to show also an interesting resemblance to cardiac muscle, 

 in the fact that each contraction is followed by a' temporary diminution in 

 irritability and conductivity ; but this important property, which in the case of 

 the heart has been so useful in explaining the rhythmic nature of its contrac- 

 tions, has not been demonstrated for all varieties of plain muscle occurring in 

 the body. 



A general property of plain muscle which is of great significance in explain- 

 ing the functional activity of this tissue is exhibited in the phenomenon of 

 " tone." By tone or tonic activity as applied to muscle-tissue is meant a con- 

 dition of continuous contraction or shortening which persists for long periods 

 and may be slowly increased or decreased by various conditions affecting the 

 muscle. Both striated and cardiac muscle exhibit tone, and in the latter at 

 least the condition is independent of any inflow of nerve-impulses from the 

 extrinsic nerves. Plain muscle exhibits the property in a marked degree. The 

 muscular coats of the alimentary canal, the blood-vessels, the bladder, etc., are 

 usually found under normal circumstances in a condition of tone which varies 

 from time to time and differs from an ordinary visible contraction in the slow- 

 ness with which it develops and in its persistence for long periods. Such con- 

 ditions as the reaction of the blood, for example, are known to alter greatly 

 the tone of the blood-vessels, a less alkaline reaction than normal causing 

 relaxation, while an increase in alkalinity favors the development of tone. 

 Tone may also be increased or diminished by the action of motor or inhibitory 

 1 Pfliiger's Archivfur die gesammte Physiologic, 1869, Bd. 2, S. 243. 



