THE PHENOMENON OF CONTRACTION. 59 



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 observed. In the mammalia the function of 

 the ciliated epithelium is supposed to be entirely mechanical, 

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

 ducts 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 protect 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 adequately 

 explain one will doubtless be applicable to the other. Many 

 interesting facts have been established regarding ciliary move- 

 ments. 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 per- 

 forming 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 

 the cell itself; the cilia of a single completely isolated cell may 

 continue to contract vigorously. The movement may continue 

 for several days after the death of the individual, thus again showing 

 the physiological independence of the structure. The ciliated cells 

 may conduct a stimulus or impulse to other cells even after its 

 own cilia have lost their contractility. This fact is particularly 

 significant in general physiology, as it aids in showing that the 

 property of conductivity which is exhibited in such high degree 

 by nerve fibers is possessed to a lower degree by other tissues. 



