AN INTRODUCTION TO ANIMAL BIOLOGY 



43 



Ciliated epi- 

 thelial cells from the 

 trachea of a dog. The 

 hair-like structures on 

 ends of cells project 

 into cavity of the 

 trachea. They have 

 constant lashing 

 movements. (After 

 Sharpey.) 



the primary units composing the body. These statements 

 are also true of all higher animals and of the human body. 



41. Life-Activities in Cells : Proto- 

 plasm. Since the frog's body is com- 

 posed primarily of cells, we are led to 

 infer that the life-activities ( 29) are 

 located in the individual cells. For 

 example, the shortening of a muscle 

 when it contracts and moves is the result 

 of the combined shortening of the thou- FIQ 16 

 sands of cells of which the muscle is com- 

 posed. It appears, then, that the living 

 active substance is located in the cells of 

 the body. The technical word for living 

 matter is protoplasm. It is not known 

 how much of the cell-substance is living ; 

 but it is certain that the protoplasm is 

 the basic or essential substance in both nucleus and cell-body. 



The "life" of the 

 frog, then, is not 

 limited to any one 

 organ, such as the 

 heart, the brain, or 

 the lungs ; rather it 

 is located in every 

 living cell of the 

 body. 



In order to under- 

 stand better that pro- 

 toplasm is living and 

 active, the micro- 



FIG. 17. Fibers of connective tissue, a, b, white SCOpe may be used to 



fibers in bundles ; c, elastic fibers. These fibers examine leaflets of 

 belong to inter-cellular substance. Numerous j. , 



small cells are present between the fibers in ' 



connective tissue (see c in Fig. 18) . plants in which the 



