THE PHENOMENON OF CONTRACTION. 55 



and in the Eustachian tube and part of the middle ear. Similar 

 cells are found lining the ventricles of the brain and the central 

 canal of the cord. The cilia in this latter position have been demon- 

 strated to be motile in the frog, but whether this is true for the mam- 

 mal has not been shown. So also in the neck of the uriniferous tubule 

 ciliated cells are said to occur, but whether they are motile or not has 

 not been demonstrated. In the internal ear and the olfactory mucous 

 membrane the so-called sense cells are also ciliated, but here at least 

 the cilia are probably not motile. Ordinarily each ciliated epithelial 

 cell carries a bunch of cilia, all of which contract together, but 

 motile protoplasmic prolongations of the cell may occur singly, as 

 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 found in mam- 

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

 contracted quickly in one direction, so as to take 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 in 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 definite 

 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 

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

 and in this latter organ their motion is supposed to guide the 

 spermatozoa from the uterus toward the oviducts, that is, 

 the resistance offered to the motile spermatozoa guides their move- 

 ments. 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 adequatelv 



