CHAPTER IV 

 THE WORK OF MUSCLES AND GLANDS 



WE cannot enter upon a description of the alimentary 

 canal and its activities until we have devoted some space 

 to the physiology of contraction and secretion. Movement 

 is the most familiar manifestation of animal life. When 

 visible to the naked eye it is the expression of the shorten- 

 ing of elongated units cells or fibers associated to form 

 contractile tissues. In the human body there are three 

 principal kinds of these tissues. The obvious external 

 movements of the limbs and the features, the act of 

 breathing, etc., are produced by what we call the skeletal 

 muscles. Contractile tissue of another order forms the 

 walls of the heart and furnishes the power for its beating. 

 A third kind occurs in the walls of the alimentary tract, in 

 the blood-vessels, and elsewhere. 



The term skeletal applied to a type of contractile tissue 

 implies relationship to the bones. It is easy to see that 

 external movements are made effective through the connec- 

 tion of the muscles which produce them with bones acting 

 as levers. In some instances the term is a misnomer, for 

 there are some small muscles histologically like the rest 

 which do not act upon bones. This is clearly the case 

 with the ring-like band which surrounds the mouth and 

 by its contraction puckers the lips as in whistling. The 

 large and conspicuous muscles are attached, usually at 

 both ends, to the bones. We can generally observe that 

 one end is more freely movable than the other. The com- 

 paratively fixed end is called the origin of the muscle, the 

 end more subject to movement is its insertion. 



What is called a skeletal muscle is a bundle in which 

 we can distinguish an active and a passive part. There are 



36 



