628 A MANUAL OF PHYSIOLOGY 



to change its place and to take in and expel food and foreign 

 bodies. The maximum velocity of the amoeboid movement has 

 been reckoned at 0-008 millimetre a second. Stimulation with 

 the constant current or 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 influences, 

 or stimuli, by manifestations of the peculiar kind which we 

 distinguish as vital or physiological. Many other unicellular 

 organisms and the chief varieties of the white blood-corpuscles 

 behave like the amceba ; and we have already dwelt upon some 

 of the important functions fulfilled by such amoeboid move- 

 ment 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 JJL 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 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 physiological salt 

 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 

 of cells do not move at the same time ; the motion spreads from 

 cell to cell in a regular wave. The energy of ciliary motion 

 may be considerable, although far inferior to that of muscular 

 contraction. The work which cilia are capable of performing 

 can be calculated by removing the membrane, fixing it on a 

 plate of glass, cilia outwards, putting weights on the glass plate, 



