THE PHENOMENON OF CONTRACTION. 55 



is illustrated in the spermatozoa, for instance, and in many of the 

 protozoa and plant cells. In the lower forms of life cilia play 

 obviously a very important role in locomotion, the capture of food, 

 and respiration, and their form and manner of movement vary 

 greatly. The form of movement or manner of contraction was 

 formerly described under four heads, the hook form, the pendular, 

 the undulatory or wave-like, and the funnel form or infundibulary. 

 With the exception of the spermatozoa, the cilia met in mam- 

 mals show the first form of contraction. The little processes are 

 contracted quickly in one direction, so as to form a hook shape, 

 and then relax more slowly, the relaxation taking several times 

 as long as the contraction. The whole movement is rhythmical and 

 very rapid. The cilia of the epithelium of the frog's pharynx and 

 esophagus, which have been the most frequently studied among 

 the higher animals, contract, according to Engelmann, at the rate 

 of 12 times per second. When a field of epithelium is observed 

 under the microscope the contractions pass over it in a defi- 

 nite direction, but so rapidly that the eye is not able to analyze 

 them, one obtains the impression simply of a swiftly flowing current. 

 As the cilia begin to die, their movements become less rapid, and 

 the nature of the contractions and their progress from cell to cell 

 can be satisfactorily determined. In the mammalia the function 

 of the ciliated epithelium is supposed to be entirely mechanical, 

 that is, they move along substances lying upon them. In the ovi- 

 ducts they move or help to move the ovum toward the uterus, 

 and in this position, moreover, their motion is supposed to 

 guide the spermatozoon from the uterus toward the oviducts, that 

 is, the resistance offered to the motile spermatozoon guides its 

 movements. So in the respiratory passages foreign particles of 

 various sorts, together with the secretion of the mucous glands, 

 are moved toward the mouth, the effect being to free the air- 

 passages from obstruction. The contraction and relaxation of the 

 cilia are assumed to be phenomena of essentially the same order 

 as those exhibited by the muscle tissue. A theory that will ade- 

 quately explain one will doubtless be applicable to the other. 

 Many interesting facts have been established regarding ciliary 

 movements. The contractions of the cilia in any given field 

 the trachea, for instance follow in a definite sequence and are 

 co-ordinated. The waves of contraction progress in a definite 

 direction. This fact increases greatly the effectiveness of the cilia 

 in performing work. Thus, in spite of their extremely minute size 

 it is estimated that an area of a square centimeter is capable of 

 moving a load of 336 gms. The contractions are automatic, 

 that is, the stimulus causing them is not dependent upon a con- 

 nection with the nervous system, but upon processes arising within 



