546 A MANUAL OF PHYSIOLOGY 



substance, and to retract them, and it is even able by such 

 movements to change place. Stimulation with induction 

 shocks causes the whole of the processes to be drawn in, and 

 the amoeba to gather itself into a ball. This illustrates a 

 universal property of protoplasm, excitability, or the power 

 of responding to certain external influences, or stimuli, by 

 manifestations of the peculiar kind which we distinguish as 

 vital or physiological. Certain of the white blood-corpuscles 

 behave like the amoeba; and we have already dwelt upon 

 some of the important functions fulfilled by such amoeboid 

 movement in the higher animals and in man. But a great 

 distinction between this kind of contraction and that of a 

 muscular fibre is that it takes place in any direction. 



Cilia. Cilia possess a higher and more specialized grade 

 of contractility. They are very widely distributed in the 

 animal kingdom ; and analogous structures are also found 

 in many low plants, such as the motile bacteria. 



In the human subject ciliated epithelium usually consists 

 of several layers of cells, the most superficial of which are 

 pear-shaped, the broad end being next the surface and 

 covered with extremely fine processes, or cilia, about 8 fi in 

 length, which are planted on a clear band. It lines the 

 respiratory passages, the middle ear and Eustachian tube, 

 the Fallopian tubes, the uterus above the middle of the 

 cervix, the epididymis, where the cilia are extremely long, 

 and the central cavity of the brain and spinal cord. 



Ciliary motion can be very readily studied by placing a 

 scraping from the palate of a frog, or a small portion of the 

 gill of a fresh-water mussel under the microscope in a drop 

 of normal saline solution. The motion of the cilia is at 

 first so rapid that it is impossible to make out much, except 

 that a stream of liquid, recognised by the solid particles 

 in it, is seen to be driven by them in a constant direction 

 along the ciliated edge. When the motion has become 

 less quick, which it soon does if the tissue is deprived of 

 oxygen, it is seen to consist in a swift bending of the cilia 

 in the direction of the stream, followed by a slower recoil 

 to the original position, which is not at right angles to the 

 surface, but sloping streamwards. All the cilia on a tract 



