AN INTRODUCTION TO ANIMAL BIOLOGY 



57 



necessary it is that all the organs concerned in the nutrition 

 of the body should work together or cooperate. To accom- 

 plish this, there are organs which cause the coordination (work- 

 ing together) of all the organs of the body. These are the 

 nervous organs of the body the brain, spinal cord, and nerves. 

 55. Nervous Organs for Coordination. If we touch a 

 living frog anywhere on its skin, various muscles of the body 

 will contract, causing the animal to jump. A similar result 

 comes from suddenly thrusting 

 a stick before the frog's eyes, or 

 from making a loud noise. The 

 same thing happens when the 

 frog sees food, such as a worm ; 

 the muscles contract so that the 

 frog jumps and seizes the worm. 

 These are examples of coordi- 

 nation between the external or- 

 gans (skin, ears, and eyes) and fibers, 

 the muscles which move the 

 body. Likewise, whenever any 

 change takes place in any inter- 

 nal organs, coordinated changes in other organs, as in the beat- 

 ing of the heart mentioned above, are caused by the nervous 

 organs. The activities of the nervous organs are due to 

 the combined working of the nerve-cells (Fig. 20) and their 

 fibers which connect them with various organs of the body. 

 A large part of the nervous organs consists of connective 

 tissues surrounding the nerve-cells and their fibers. 



A nerve-cell and its 

 n, nucleus; ac, main fiber 

 (axis cylinder). The other fibers 

 are shorter and with many 

 branches near the cell. 



DEVELOPMENT OF THE FROG: INTRODUCTION TO 

 EMBRYOLOGY 



56. Reproduction. So far in considering the work of the 

 organs of the frog, we have given attention to the organs 

 which are necessary for the life of the individual frog, and 



