THE MUSCLES. 363 



Every muscle is composed of thread-like fibers, running in 

 the direction in which it is intended to act. These fibers form 

 the bulk of the muscle ; but they are bound and held together 

 by fine fibers of tendon, running through the entire bulk of the 

 muscle, and spread over its body, forming the sheath of the 

 muscle. At the extremities of the muscle, where it is to be 

 fastened or attached to a bone or cartilage, these tendonous 

 fibers are collected together, and form what is known as the 

 tendon of the muscle, and which is fastened to the bone or 

 cartilage. Some tendons are round, and hard, gristly sub- 

 stances; others are broad and thin. Toughness and strength 

 are the properties of all tendons. One end of the muscle is 

 said to be its origin, the other its insertion. 



The power of a muscle depends on the number of muscle 

 fibers it is composed of. Some muscles have but a very few 

 fibers, and consequently are almost white. 



The spaces between the muscles is mainly filled up by cellu- 

 lar tissue, in which the fat is deposited. It also dips into and 

 is interspersed through the body of many muscles. It is in 

 the little cells of this whitish, spongy tissue that the fat is de- 

 posited or formed. The fat thus interspersed through the 

 muscles serves to lieep them oiled and soft. 



Every muscle has the power of contracting, or drawing up, 

 and Off relaxing, or becoming loose. Most muscles are under 

 the control of the will of the animal, as those of the limbs. 

 They are called voluntary. Some act independently of the 

 will, and are called involuntary muscles, as the heart, etc. It 

 is by this power of the muscles that all movements of the ani- 

 mal are performed. In proportion to the size of the muscle 

 will be the power with which it is capable of contracting; but, 

 however powerful may be the muscles of the horse, if the bones 

 to which they are attached are not j^roperly formed for the 

 muscles to act with advantage, their power will be spent with- 

 out exhibiting either great strength or quick action in the 

 animal. 



