PHENOMENA OF MUSCULAR MOTION. 293 



is therefore called a hollow muscle. They also form one of the 

 coats of the stomach and intestines, and of the urinary bladder. 



In the muscles on the trunk and limbs, their arrangement is 

 very various, being rectilinear, penniform, radiated, &ic. 



There are a great many short fibres, with an oblique direction, 

 in some muscles of small volume, which have therefore creat 

 power and little motion, as in the semimembranosus. 



— Contraction, though the only power that muscles appear 

 to exercise, is found likewise existing to some extent, in other 

 tissues of the body, where some effort and resistance is required 

 in the performance of their functions, without the necessity 

 of that perfect antagonism of action which muscles usually 

 establish. There appears, in fact, to be a regular gradation in 

 the changes of the condition of the muscular fibre. The mus- 

 cles of animal life in man are the most fully developed, most 

 highly colored, and enjoy to the fullest extent, the powers of 

 contraction. Their only vital action is that of contraction, 

 which as has been before explained, (see page 291,) causes 

 their ends to approach each other, by moving one or the other 

 of the bones, to which the two ends of the muscles are attached. 

 One of the bones is usually more readily moved than the other, 

 and that is the action of the muscle as usually set down in 

 books. But the student will do well to impress upon his mind, 

 that this is not the only movement which the muscle can effect, 

 and that if the part which it usually moves, becomes from 

 some adventitious cause more solidly fixed than the other, 

 as from a weight attached to it, or the opposing action of other 

 muscles, the contraction of the muscle will produce a move- 

 ment of the part, at which its other extremity is inserted. In 

 this way the action of muscles is beautifully varied, and very 

 complicated and useful movements are produced in the body, 

 by what seems a very simple arrangement of the muscles. 

 Thus the action of the great pectoral and latissimus dorsi mus- 

 cles is usually to pull down the arms when they have been 

 elevated by other muscles. But if the arms are thrown 

 upwards, and the hands grasp some place above, as the limbs 

 25* 



