spiral Structure of Muscle. 181 



tremities of the cilia n; but it instantly i-eturns to its previous 

 state, to be immediately afterwards depressed again as before ; 

 and so on*. Thirdly. You sometimes meet with states in which 

 this depression of some of the cilia m is permanent. See m' 

 in the same figure. Fourthly. As already mentioned, the ciha m 

 in different individuals present very different states, figs. 31, 33, 

 33, 34, 39, 40. Their lengths differ, some being very short ; 

 sometimes they are straight, sometimes curved ; sometimes they 

 are found moving, sometimes motionless. The movements are 

 generally quite irregular. There is nothing like a common pur- 

 pose in them ; certainly no combination for the production of a 

 current. In some they are such as to suggest the idea of efforts 

 to become unbent at their extremities, fig. 32 ; and in others no 

 longer bent at their extremities, the movements seem made for 

 the purpose of becoming elongated, fig. 40. You sometimes 

 meet with the two last-mentioned states in the same bar, fig. 39. 

 In short, these different appearances and movements evidently 

 denote different degrees of development. Not until they reach 

 the state in fig. 36 can the cilia m be said to have attained ma- 

 turity, and to exhibit a common purpose in their movements. 

 But even then their movements are not so vehement as to be 

 likely soon to wear them out. Why, then, are they so constantly 

 renewed ? The fact is, that by flexion at the base, the cilia m 

 (see fig. 36) pass, one after another, into the vehemently vibrating 

 cilia n, which they succeed as a later generation. For this pur- 

 pose they are formed, and then for the first time do they perform 

 really eflicient action. Thus it was that the author was led to 

 his position No. 2. 



In his third conclusion, he stated that the cilia o in fig. 36, 

 &c., hitherto either overlooked or held to be identical with the 

 cilia n, are really not so,— they are the counterpart thereof. This 

 will be immediately made clear if attention be paid to their origin 

 and the function they perform. 



The roots of these two lines of cilia are separated by a broad 

 pellucid space, fig, 36 h, in which are no cells such as those [p) 

 giving origin to the cilia in question. The cilia of the two fines, 

 proceeding from opposite sides of the pellucid space, arch over it, 

 their extremities meeting in the middle line, where they, alter- 

 nating with one another, like the fingers of the two hands, form 



* The movements of the ciiia ni are described by Sharpey merely as 

 follows :— " The more opake cilia, or those of the exterior range, ajjpear 

 and (lisap])ear by turns, as if they were continually changing from a liori- 

 zontal to a vertical direction and back again."— L. c. p. 622. And the 

 author says, he is not aware that any bther author has given more exact 

 mformation concerning them. 



