AN INTRODUCTION TO THE VERTEBRATES 237 



between a living animal and a machine like a clock, but just as 

 a boy takes a clock apart to see what makes it go, so if we wish to 

 know what enables an animal to perform its various activities, 

 we must dissect it and examine its various parts. 



Organs and Systems of Organs. — The principal parts of an 

 animal, such as the eye, the stomach, or the arm, we call organs. 

 Many organs are usually necessary for the performance of a 

 single function; for example, the proper digestion of food in a 

 complex animal requires a large number of organs collectively 

 known as the alimentary canal and its appendages. These 

 organs constitute the digestive system. Similarly, other sets of 

 organs are associated for carrying on other functions. The 

 principal systems of organs and their chief functions are as 

 follows : — 



(1) Digestive system — Digestion and absorption of food. 



(2) Circulatory system — Transportation of food, oxygen, 

 and waste products. 



(3) Respiratory system — Taking in oxygen and giving off 

 carbon dioxide. 



(4) Excretory system — Elimination of the waste products 

 of metabolism. 



(5) Muscular system — Motion and locomotion. 



(6) Skeletal system — Protection and support. 



(7) Nervous system — Sensation and correlation. 



(8) Reproductive system — Reproduction. 



(1) The digestive system has for its functions the changing 

 of solid food into liquids and the absorption of these liquids into 

 the blood. This system consists usually of a tube, the alimen- 

 tary canal, with an opening at either end of the body. Con- 

 nected with this tube are a number of glands. Solids taken in 

 as food are usually broken up in the mouth, where they are mixed 

 with juices from the salivary glands ; the mixture then passes 

 through the oesophagus into the stomach, where chemical diges- 

 tion, aided by secretions from the gastric glands, takes place; 

 it then enters the intestine, which absorbs the dissolved material 



