312 THE ANIMAL KINGDOM. 



and the other of a pin's head to the most powerful stroke 

 of the arm, are performed by the contraction of the muscles. 

 But there are several actions which would at first appear to 

 be produced by elongation instead of contraction, such as 

 the protrusion of the tongue, the winking of the eyelids, 

 and the closure of the lips ; but the first of these is produced 

 by the contraction of a muscle which is attached at one 

 extremity to the under part of the root of the tongue, and 

 at the other to the inner part of the lower jaw, which, when 

 it contracts, draws the whole tongue forwards, causing a, 

 part of it to protrude ; the eyelids and lips are closed by 

 circular muscles surrounding them, which, by contraction, 

 draw together those parts. 



The muscles form the flesh of an animal, or that part 

 which is of a red colour, and which is called lean (in contra- 

 distinction to fat), they are composed of fibres, each fibre 

 made up of a number of fibrils, and the whole bound up 

 together by means of a fine membrane, called areolar tissue. 

 Each of these fibrils consists of a number of cells pressed 



FIG. 77.-FH3RILS OF VOLTTNTAEY 

 MUSCLE, MAGNIFIED. 



closely together and having a peculiar bearded appearance 

 (figs. 77, 78). It is by the sudden approximation and flat- 

 tening of these cells that the muscle is shortened during con- 

 traction, but the contraction of each fibril is only for an 

 instant, and then a relaxation and elongation of the cells takes 

 place, which is followed by a second contraction, and so on. 

 Now, the way in which a muscle keeps in a state of permanent 

 contraction is this : the various fibrils or strings of cells relieve 

 each other instead of all contracting and all relaxing toge- 

 ther, and in this way a part only of the fibrils are in a state 

 of contraction at any given time, while the others are 

 relaxed, and as one begins to relax another contracts, and 



