AN INTRODUCTION TO ANIMAL BIOLOGY 37 



Circulation organs: heart, arteries, veins, capillaries, 

 lymph- vessels, and spleen. 



Excretory organs: kidneys, ureters, bladder, cloaca, 

 lungs, and skin. 



Nervous organs : brain, spinal cord, nerves, eye, ear, and 

 nose. 



Supporting organs : skeleton (bones and cartilages) . 



Muscle organs : muscles of body-wall, of limbs (for locomo- 

 tion), and of internal organs (heart, stomach, etc.). 



Reproductive organs : ovaries, spermaries, ducts of ovaries 

 and spermaries, and fat-bodies (peculiar to frog and its 

 relatives or allies). 



THE TISSUES OF THE FROG : INTRODUCTION TO 

 MICROSCOPIC STUDY 



38. We have found that the frog's body is composed 

 of many parts which we call organs, examples of which are 

 skin, liver, stomach, heart. We shall now examine more 

 carefully the structure of these organs. 



(L) Examine a frog's leg. First on the outside, notice the skin. 

 Remove the skin, and the muscles are brought into view. Compare 

 a piece of skin with a piece of muscle. Is there any difference? 

 In separating the skin from the muscle, or the muscles from each 

 other, notice fine but strong threads binding them or connecting 

 them together. Notice bright, glistening bands (called tendons) 

 which unite muscles to bones. Next, examine the nerves which lie 

 between the large muscles. Scrape the muscle from the bone and 

 attempt to cut into it about the middle and also at the rounded 

 ends. Is there any difference ? 



In thus examining the frog's leg we find that it is com- 

 posed of several distinct kinds of building materials. These 

 are tissues. The skin is an example of covering and protec- 

 tive tissue which is called epithelium or epithelial tissue. 

 The strong threads binding skin to muscle, muscles to each 

 other, and (in form of tendons) muscles to bones, is known as 



