WILLIAMS : PHENOMENA OF MUSCLE-CONTRACTION. 47 



resembles, evidently, the spontaneous movement of protoplasm. 

 But the work done represents a large amount of physical labour 

 and is much greater than that performed by ordinary muscular 

 contraction. The absolute force of movement is the weight 

 that can be distinctly moved when covering a surface of one 

 centimetre. This has not been estimated, so far as I am aware, 

 in the mollusca, but the lowest value for the pharyngeal mucous 

 membrane of the frog, an animal in which the cilia are much 

 shorter, is 3*36 grammes. When the estimation is made for the 

 mollusca we shall, therefore, expect a much greater estimate 

 than this one. The rapidity with which the strokes succeed 

 one another is very great — when moving at their fastest the 

 movement of the whole resembles a stream of running water, 

 and when moving more slowly it recalls a field of corn shaken 

 by the wind. A temperature between o°c and 4o°c favours the 

 movement, as also does the presence of oxygen, water, or air. 

 Ozone, oxygen under a pressure of eight atmospheres, alkalies, 

 acids, bile, dilute saline solutions, chloroform, amyl nitrite, 

 ether, hydrogen and carbonic dioxide arrest or retard the move- 

 ment. During movement electricity is set free, for a current has 

 been observed passing from the superficial to the deeper parts. 

 What the cause of the movement may be, in reality, due to has 

 been the subject of much discussion. Engelmann has brought 

 forward, what I consider, the most plausible theory, and one 

 which we must accept only tentatively for the time being as 

 something to hold on to in our work until some other still more 

 plausible explanation is advanced by our brethren-workers either 

 in this country or on the continent. He considers that each 

 cilium is composed of serially arranged particles, to each of 

 which he has given the name of an inotagmen, which, during 

 rest, are contracted with the long axis parallel to that of the 

 cilium, and, during action, spherical, the whole movement re- 

 sponding to external stimuli. That the contraction is a response 

 to external stimuli is supported by the observation of Steinbuch, 

 who found that a mechanical stimulus, insufficient to injure the 



