16 PHYSIOLOGY. 



frequently it is a movement of elevation and depression of the cilia; 

 sometimes it is like the extension and flexion of our fingers, at other 

 times a sort of wave or whirlpool-like movement. In these move- 

 ments all the cilia on the surface move in the same direction, like a 

 field of grain before the wind. Each completed movement of the 

 cilia is composed of two movements of unequal duration, the longer 

 corresponding to contraction, and the shorter to relaxation of the 

 cilia. Ciliary movements may be of a high rapidity, as many as 

 960 to about 1000 per minute, and entirely independent of the circu- 

 lation and the nervous system. These movements are able to con- 

 tinue after death as long as a day, while in frogs they have been 

 observed for many days. - 



Cilia are about 1 / 300 o mc h i n length and are able to perform 

 some work. By their movements they are able to float a cell in a 

 liquid, such as water, even though the cell and cilia are composed, in 

 a great part, of protoplasm, whose specific gravity is heavier than 

 water and naturally inclined to sink, and at the same time they pro- 

 pel the cell in some definite direction at a much faster speed than that 

 obtained by the protrusion and retraction of pseudopodia. The 

 function of the ciliated cells does not appear to be of any particular 

 importance in man except that in the trachea their movements bring 

 to the larynx foreign substances that have been inhaled into the lungs, 

 such as dust, etc., and to bring up for expectoration the thickened 

 mucus that is formed during the stages of a cold. 



A practical illustration of the effects of the protoplasmic move- 

 ments of leucocytes (white blood-corpuscles) can be observed when an 

 injury occurs to any part of the body. As a result of the injury and 

 as an attempt at repair, more blood is sent to the injured part. This 

 result, called congestion, gives to it its red color. With the additional 

 quantity of blood comes an additional number of leucocytes. They, 

 by protoplasmic movements, pass through the walls of the capillaries 

 to the seat of the injury, to take up dead portions. Sometimes bac- 

 teria lodge in the wound, which the leucocytes approach and kill by 

 ingestion, as it were, thus rendering them harmless. This process of 

 ingesting bacteria and other foreign substances is called phagocytosis, 

 and hence the leucocytes are sometimes termed phagocytes. 



The phenomenon of a leucocyte in active movement, which by 

 one-sided action of the chemical products of bacteria, as toxins, moves 

 toward (positive) or away (negative) from the bacteria, is called 

 chemotaxis. 



