222 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



trated and webbed together in all its parts with con- 

 nective tissue. In a condensed form it invests each 

 fiber ; in a loose form it lies between and connects them. 

 In a condensed form it again invests the bundles ; in a 

 loose form it lies between and con- 

 nects these also. Finally, it emerges C 

 on the surface, and in a condensed 



Fig. 142. — A, a fascicle of muscular fibers of voluntary muscle ; B, one 

 fiber broken to show its investing sheath ; C, cells of involuntary muscle. 



form invests the whole muscle in a strong sheath, and 

 in a loose form lies between the different muscles, con- 

 necting yet separating them. Add to this the blood 

 vessels to nourish and nerves abundantly distributed to 

 stimulate to contraction, and we have a good general 

 idea of the organ we call a muscle. 



The tendon consists of all the sheaths of the muscle, 

 of the bundles, and of the fibers united and continuing, 

 and also of the transformed fibers themselves. It is 

 therefore essentially fibrous in structure. It is the 

 strongest — i. e., has the greatest tensile strength of any 

 organic substance known. 



The only function of a muscle is to contract. This, 



