Specialized Cells: Muscles, Nerves and Sense Organs 111 



MUSCLE 



The simplest organs which produce motion are the flagella or 

 streamers attached to some bacteria (Fig. 18). These seem to be 

 threads or filaments of protein. They are capable of swaying from 

 side to side in a kind of wave motion and this propels the bacterium 

 through the water. Spermatozoa have a similar organ in their tails 

 which enables them to swim through the vaginal fluid until they reach 

 the ovum. These tails contain the enzymes which are required to pro- 

 duce the energy they need from the breakdown of sugar — they are in 

 fact miniature muscles. However, it is likely that they start their 

 journey with a supply of 'fuel', i.e. sugar, and when this is used up 

 they are unable to go any further. 



The muscles of the higher animals are large masses of cells which 



FIG. 18. A bacterium with flagellae (B. proteus vulg., after Weibull) 



have the power of contracting. They are often attached to the bones 

 of the skeleton by tendons and are able to exert considerable forces. 



Muscle is clearly a very complicated organ and it has been studied 

 from several points of view. These concern themselves with different 

 parts of the whole question; they are all necessary and a complete ex- 

 planation of muscle action must cover the three main problems to 

 which an answer is required. These are: 



(1) The mechanical arrangements whereby contraction occurs; 



(2) the biochemical reactions which provide the energy; 



(3) the 'activation process', usually stimulated by nerve signals, 

 which causes the muscle to contract. 



All these have been exhaustively studied and at different times the 

 emphasis has been put mainly on one or the other. For a long period 

 (1920-1940) much of the interest was in the biochemical reactions 

 which provided the energy of muscular contraction and these have 

 been fairly fully worked out. It has been established that the energy- 



