CHAP, ii.] CONTRACTILE TISSUES. 103 



contraction of the cell itself, the cilium acting merely as a minute 

 elastic rod ; and some such view as this is supported by the fact 

 that no movement has ever been observed in an isolated cilium. 

 It is difficult however to understand how the peculiar sickle-like 

 flexion of the cilium can be brought about unless the contractile 

 material is continued up into the cilium itself; and the tail of a 

 spermatozoon, which is practically a single cilium, may contract 

 even when separated from the head. 



Ciliary movement appears therefore to differ from ordinary 

 muscular contraction chiefly in the size of the apparatus concerned. 

 The movement is rapid: thus Engelmann has estimated that 

 in the frog the flexions are repeated at least twelve times in a 

 second. The movement in fact is too rapid to be visible; it 

 can only be seen at a time when exhaustion and coming death 

 have begun to retard the action; Engelmann found that he was 

 first able to count them when their rapidity declined to eight in a 

 second. 



In the vertebrate animal, cilia are, as far as we know, wholly 

 independent of the nervous system, and their movement is pro- 

 bably ceaseless. In such animals however as Infusoria, Hydrozoa, 

 &c. the movements in a ciliary tract may often be seen to stop 

 and go on again, to be now fast now slow, according to the needs 

 of the economy, and, as it almost seems, according to the will 

 of the creature ; indeed in some of these animals the ciliary move- 

 ments are clearly under the influence of the nervous system. 



Observations with galvanic currents, constant and interrupted, 

 have not led to any satisfactory results, and, as far as we know at 

 present, ciliary action is most affected by changes of temperature and 

 chemical media. Moderate heat quickens the movements, but a 

 rise of temperature beyond a certain limit (about 40C. in the case 

 of the pharyngeal membrane of the frog) becomes injurious ; cold 

 retards. Very dilute alkalis are favourable, acids are injurious. 

 An excess of carbonic acid or an absence of oxygen diminishes or 

 arrests the movements, either temporarily or permanently, according 

 to the length of the exposure. Chloroform or ether in slight doses 

 diminishes or suspends the action temporarily, in excess kills and 

 disorganises the cells. 



Migrating Cells. We have already (p. 35) urged the view that 

 an amoeboid movement of a white corpuscle is essentially a form 

 of contraction. 



All the circumstances which affect muscular contraction, heat, 

 absence or presence of oxygen and carbonic acid, &c., also affect 

 protoplasmic movements. The white corpuscles, like muscular 

 fibres, suffer rigor mortis, in which state they become spherical. 



