8 ANATOMY AND PHYSIOLOGY 



An organ is any part of the body designed for a special func- 

 tion or use; it may be composed of several kinds of tissue. An 

 organ in the interior of the body (internal organ) is called a 

 viscus (pleural, viscera). Examples, heart, lungs. 



A system is composed of a number of organs of similar structure. 

 Examples, the muscular system, the nervous system. 



An apparatus is composed of a number of organs of like or 

 different structures, so arranged and associated that their action 

 together will serve a special purpose. Example, the digestive 

 apparatus. 



Metabolism. This term is used to express in one word the 

 related processes of building up and breaking down which are 

 constantly going on in all living cells. 



The cell appropriates materials and combines them to perfect 

 itself; in the exercise of its function it uses up some portion of its 

 substance and so must be again built up, to be again pulled apart 

 in endless repetition. 



Cell action in some tissues results principally in liberating heat and in 

 body movement, as in muscles. In others it forms new compounds for other 

 cells to use for example, the liver cells form glycogen; the gastric glands 

 secrete gastric juice, etc. Again, certain cells combine waste matters to get 

 them into shape for other organs to excrete, for example, the formation of 

 urea in the liver. In this way food materials are used for different purposes 

 and worked over in different tissues until waste alone remains. 



These examples (and many more which might be given) illustrate the 

 metabolism which is constantly taking place in the body, and which will 

 often be referred to in the text. (See pp. 166, 271.) 



Structure of Glands: Since gland tissue is so important 

 a factor of vital processes, a further description is warranted. 

 It has already been stated that the epithelial layer is the active 

 agent in the formation of new substances out of material derived 

 from the blood. For the performance of this function, the 

 epithelium is disposed in organs called glands. 



The simplest gland is either a small tube, or a sac. The 

 tubular gland may be divided into two or more portions forming 

 a compound tubular gland. Tubular glands exist in the stomach, 

 intestines, skin, etc.; in the skin they are coiled or convoluted at 

 the extremity. (See Fig. 166.) A modification of the saccular 

 gland is one which is composed of many small sacs arranged like 



