NATURAL HISTORY. 



*' Look round our world ; behold the chain of Lore 

 Combining all below, and all above." Pors. 



dead and powerless. If the heart is newly separated from the body, it contracts if 

 irritated. The bowels continue their penal ultic motion after death, until they be- 

 come stiff and cold. This quality belongs absomte.y to themuscie. and exists, in some 

 cases without nervous irritability altogether hence, there is a distinction between 

 nervous sensibility and muscular irritability. The former dies immediately with 

 the animal ; the latter lives for a short time after the animal is dead. Muscles are 

 irritab'e and contractile by the inherent principle of their fibres, and are sensible by 

 the vitality communicated through their nerves. Though nerves are sensible, they 

 are not contractile, and cannot perform the functions of muscular fibres. 



61. Why do we find -muscles under a multiplicity of forms and 

 attituaes, sometimes with double, sometimes with treble tendons, some- 

 time* witft none, sometimes one tendon to several muscles, at other 

 times one muscle to several tendons ? 



The nvison for this great mechanical variety in the figure of the 

 muscles, > owing to a fixed law that the contraction of a muscle 

 shall be towards its centre. Therefore, the object for mechanism on 

 each occasion is so to modify the figure and adjust the position cf 

 the muscle as to produce the motion required agreeably with this 

 law. This can only be done by giving to different muscles a diver- 

 sity of configuration suited to their several offices, and to their situa- 

 tion with respect to the work which they have to perform. 



62. The illustration of this principle is as follows : A is the tendinous organ ; B 



the tendinous insertion, and the 

 muscular fibres run obliquely be- 

 tween them. This obliquity of 

 the fibres is almost universal in 

 the muscles of the limb, and the 

 effect is very important. If we 



pull obliquely uron a weight, we sacrifice a great deal of power. For what ad- 

 vantage, then, is power resigned in the muscle I If we wish to draw a tLiiig 



towards any place with the least 



force, we must pull directly in the 



line between the object and the 



place ; but if we wish to draw it as 



quickly as possible, without any 



regard to the loss of force, we must 



pull it obliquely by drawing it in 



two directions at once. Tie a 



string to a stone A, and draw it 



straight towards you at C with one 



baud ; then make a loop on ano- 



ther string, and running the first 



