THE MUSCULAR SYSTEM 53 



very minute, but being arranged in compound bundles of 

 successively larger size, we have ultimately only a single 

 bundle in a muscle. 



The essential characteristic of a muscle is its contractil- 

 ity it contracts in length, dilates in breadth, its body li 

 becoming firm and rigid. 



Muscles are said to originate at the point where they 

 have the most fixed attachment. By insertion is under- 

 stood the opposite end, or that termination or attachment 

 that is more movable ; that is, the end of the muscle that 

 most moves the bone, or portions to which it is attached. 

 The muscles are abundantly supplied with blood-vessels 

 and nerves. They are frequently the seat of rheumatic 

 disorders, as well as of inflammation. 



The number^ of muscles in the human body is commonly 

 estimated at 405; with the exception of 9, they all occur 

 in pairs. 



The muscles vary widely in size and in form. In the 

 limbs they are of considerable length; this is true es- 

 pecially of the more superficial muscles, the deep ones 

 generally being broad. They surround the bones, and 

 form an important protection to the various joints. In 

 the trunk the muscles are broad, flattened, and expanded, 

 forming the parietes of the cavities which they enclose. 

 For purposes of description, therefore, muscles are termed 

 long, broad, short, etc. 



Bach muscle is invested externally by a thin cellular 

 layer, forming what is called its sheath : this not only 

 covers its outer surface, but penetrates its interior, in the 

 spaces between the fasciculi, surrounding these, and serv- 

 ing as a bond of connection between them. 



The muscles are connected with the bones, cartilages, 

 ligaments, and skin either directly or through the medium 

 of fibrous structures called tendons or aponeuroses. 

 (See pages 46 and 47.) 



