124 PHILOSOPHY or ZOOLOGY. 



sue, consist of fibrin, whose properties have been already 

 detailed. The tendons, on the other hand, consist chiefly 

 of o-elatine. Hence, when meat is boiled, the tendons are 

 dissolved by the warm water, and the muscles separate 

 readily from the bones. Although the presence of tendons 

 in the muscles of testaceous mollusca cannot be perceived 

 by the eye, yet, as boiling water destroys their connection 

 \\ith tin- shells, the presence of gelatine, and hence of ten- 

 don, may be inferred. In quadrupeds which leap much, 

 as the Jerboa, and in birds which walk much, the tendons 

 become in part ossified ; so that the points of support of 

 the muscle are thereby increased. 



Muscles are said to be either simple or compound in 

 their structure. In the simple muscles, the fibres have a 

 similar direction ; and are either formed into a long round 

 bundle, thickest in the middle, termed ventriform ; or they 

 proceed from an extended base, and converge to a small 

 tendon, when they are termed radiated ; or resemble a fea- 

 ther, and are called pennifonn. In these last, the muscular 

 fibres are arranged like the barbs of a feather, along a 

 middle line of tendon, resembling the shaft. In the com- 

 pound muscles, the bundles of fibres and tendons are vari- 

 ously interwoven. 



Muscles arc the most active members of the animal frame. 

 They alone possess the power of irritability, and execute 

 all the motions of the body. The causes which excite them 

 to action, may be reduced to two kinds. In the first, the 

 will, through the medium [of the nerves, excites the irrita- 

 bility of the fibres; and in the second, the action is produ- 

 ced by the application of external objects, either directly, or 

 by the medium of the nerves. 



The division of the muscles into voluntary and involun- 

 tary, is sufficiently accurate to convey distinct ideas of these 



