CH. vin.] RHYTHMICALITY. IO; 



which has become almost proverbial, since it possesses the same 

 power to a marked degree. This function is a protective one; the 

 animal approximates in colour that of its surroundings, and so 

 escapes detection. 



In the retina we shall find a layer of pigment cells (fig. 131), 

 the granules in which are capable of moving in the protoplasm in 

 a somewhat similar way ; the normal stimulus here also is light. 



3. Ciliary movement ; here we have a much more orderly 

 movement which has already been described (see p: 31). 



4. In Vorticellae, a spiral thread of protoplasm in their stalk 

 enables them by contracting it to lower the bell at the end of the 

 stalk. 



5. In certain of the higher plants, such as the sensitive and 

 carnivorous plants, movements of the stalks and sensitive hairs 

 of the leaves occur under the influence of stimuli. 



6. Muscular movement. This for the student of human 

 physiology is the most important of the series ; it is by their 

 muscles that the higher animals (man included) execute the 

 greater number of their movements. 



If we contrast together amoeboid, ciliary and muscular move- 

 ment, we find that they differ from each other very considerably. 

 Amoeboid movement can occur in any part of an amoeboid cell, 

 and in any direction. Ciliary and muscular movement are 

 limited to one direction ; but they are all essentially similar, 

 consisting of the movement of hyaloplasm in and out of spongio- 

 plasm ; it is the arrangement of the spongioplasm that limits and 

 controls the movement of the hyaloplasm (see also p. 91). 



Rhythmicality. In some forms of movement there is not only 

 order in direction, but order in time also. This is seen in ciliary 

 movement, and in many involuntary forms of muscular tissue, such 

 as that of the heart. Here periods of contraction alternate with 

 periods of rest, and this occurs at regular intervals. Under the 

 influence of certain saline solutions,* voluntary muscles m'ay be 

 made artificially to exhibit rhythmic contractions. 



A familiar instance of rhythmic movement in the inorganic 

 world is seen in a water-tap nearly turned off but dripping ; 

 water accumulates at the mouth of the tap till the drop is big 

 enough to fall ; it falls, and the process is repeated. If, instead 

 of water, gum or treacle, or some other viscous substance is 

 watched under similar circumstances, the drops fall much more 



* Biedermann's fluid has the following composition : Sodium chloride 

 5 grammes, alkaline sodium phosphate 2 gr., sodium carbonate 0.5 gr., 

 water i litre. If one end of the sartorius of a curarised frog is dipped into 

 this fluid, it contracts rhythmically in a manner analogous to the heart 



