ELEMENTARY VITAL PHENOMENA 247 



high. It is easy from this to compute the enormous labour per- 

 formed by this organ during the whole life of a man. The muscle 

 is the most perfect dynamic machine known. 



Ciliary movement, finally, is no less wide-spread than the two 

 other forms of contraction. The infusorian that bustles about 

 actively in the water of a puddle moves by the strokes of flagella 

 or cilia. The spermatozoon that in fertilisation strives after union 

 with the egg-cell is driven forward by the vibrations of its flagellum. 

 The cells of ciliated epithelium that line air-passages keep the 

 mucous membrane clean by their activity, and by the rhythmic 

 beating of their cilia shove to the outside foreign bodies that have 

 come into the passages in swallowing. The host of Infusoria is 

 numberless, flagellated spermatozoa are wide-spread among both 

 plants and animals, and there is scarcely a group of animals whose 

 bodies do not possess in some spot ciliated epithelium. 



Like muscular movement, ciliary movement is co-ordinated 

 i.e., the motile particles are shifted in a definite direction. This 

 is rendered possible by the fact that the contractile elements, as 

 in the muscle-cell, are developed as constant differentiations of the 

 cell-protoplasm in the form of short, hair-like appendages of the 

 cell-body. According as the cell possesses one cilium or a few long 

 ones, or many short ones, the term flagellated cell (Fig. 107, 

 0, D, JE), or ciliated cell (Fig. 107, A, B), is employed. The 

 phenomena of ciliary motion result from the performance of 

 vibratory movements by the flagella or cilia. 



The following are the chief characteristics of ciliary motion. 

 In contrast to most forms of muscular motion, which with few 

 exceptions (Infusoria, heart-muscle) come about only as the result 

 of external impulses from the nervous system, ciliary motion is 

 automatic, i.e., the impulses that lead to it arise in the cilia them- 

 selves ; there is no known case in which the motion is at all under 

 the influence of the nervous system. It has been determined by 

 vivisection experiments 1 that the cause of the motion is seated in 

 the protoplasm of the cell-body, for, if the isolated cilia possess 

 absolutely no protoplasm at their bases, they are wholly motionless. 

 Further, most cases of ciliary motion are distinguished by their 

 rhythm, for except in certain flagellate and ciliate Infusoria the 

 cilia contract always at regular intervals, at least during pronounced 

 activity. The vibrations become irregular only during the trans- 

 ition to the resting-stage or under the influence of external factors. 

 Finally, a third characteristic, which belongs only to multiciliated 

 cells, is the metachronism of the motion of the individual cilia. 

 The individual cilia of a row, beginning at one end, contract in 

 exactly the same rhythm and succession, so that every beat of the 

 first cilium is followed by a beat of the second, then of the third, 

 1 Of. Verworn ('90, 2), 



