i Q4 PHILOSOPHY GF ZOOLOGY. 
sue, consist of fibrin, whose properties have been already 
detailed. The tendons, on the other hand, consist chiefly 
of gelatine. 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 
hy the eye, yet, as boiling water destroys their connection 
with the 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 penniform. 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 are the most active members of the animal frame. 
They alone possess the power of irritability, and execute 
all the motions of the bedy. 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 ef 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 inzolun- 
tary, 1s sufliciently accurate to convey distinct ideas of these 
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