STRUCTURE OF THE VERTEBRATES 



105 



body with processes at either end. One set of fibers (the den- 

 drites) receive impulses which are passed through the cell body 

 and out to nerve cells or other tissues through the long axone. 

 These tissues are the coordinating mechanism of the body. 

 The processes may be re-developed, but the destroyed cells can- 

 not be replaced. 



The student cannot be told too often that the organism is a 

 unified mechanism and depends upon the proper coordination 

 and function of all its parts. Proper function depends upon 

 proper balance between tissues and organs. This interdependence 



^ Epithelium 

 Muscle 



tMJbnnective 

 tissues "^^ 



Secretory 

 epithelium 



Wall of Artery 



Salivary Gland 



Fig. 46. Organ Formation, showing different tissues combined in a single 



organ. 



of the tissues can be illustrated with any organ of the body. 

 We speak of muscle tissues, but a muscle is an organ, not a tis- 

 sue. The muscle tissues are the contractile elements, but for 

 their proper growth and function they must be bound securely 

 into a functional unit by connective tissues; the impulses which 

 cause them to contract must pass over the nerve fibers which 

 innervate them; and for food and oxygen they depend upon 

 the fluid tissues to transport these substances to them. And the 

 thin epithelial walls of the capillaries permit the plasma to pass 

 out to the muscle fibers. 



Similarly, every organ of the body is made of several types 

 of tissue. The blood vessels are composed of muscular and con- 

 nective tissues, lined by epithelia, and controlled by nerve stim- 



