STRUCTURE OF MUSCLES. 127 



ces from the joints. The positions assigned them are not al- 

 ways the most favourable for the movement of the levers ; 

 but they could not be otherwise, without altering the external 

 shape, and giving to it an appearance the very reverse of those 

 round or tapering forms so commonly exhibited in organiz- 

 ed bodies. But the disadvantages attending the obliquity 

 of the muscles, with respect to the bones which they are 

 destined to move, are abundantly compensated by other 

 means. The heads of the bones are frequently larger than 

 the body to which the tendons are attached ; so that these, 

 by passing over the convexity to the place of their insertion, 

 form a more obtuse angle with the lever, than if the head 

 did not exist. The origin and insertion of a muscle are 

 frequently at some distance ; and the muscle passes through 

 holes in other bones, or is bound down by annular liga- 

 ments ; so that the motion is performed without destroying 

 the proportion of the part. Although muscles, which thus 

 move the bones as so many levers, may be compared to 

 ropes in the effects which are produced, there is a remark- 

 able difference in the manner of their action. As the 

 muscles contract in length in every part, and, by the con- 

 traction, change their shape and become more rigid and an- 

 gular, every bundle may be said to assist another ; so that 

 in the very muscle itself, there may be formed both levers 

 and fulcra ; thus giving to it powers, which, were it acting 

 like a cord, it could not have in consequence of its posi- 

 tion. 



Let us now take a view of the muscles, as concerned in 

 the production of the different motions exhibited by ani- 

 mals. These are exceedingly various in their extent and 

 duration, and in the organs employed to perform them. 



It is obvious, that as animals are exposed to the vicissi- 

 tudes of the elements, the fluctuation of the atmosphere and 



