THE PARTS OF BODIES 



outer portion called the cell wall which incloses a mass of 

 substance known as protoplasm (pro'td-plaz'm : Greek, 

 protos, first; plasma, form). The protoplasm is made up 

 of a substance called cytoplasm (sl'tu-plaz'm : Greek, kytos, 

 hollow place ; plasma, form), in which is held a saclike 

 body, the nucleus (nu/kle-us : Latin, nucleo, to become 

 hard). The nucleus usually contains one or more separate 

 bodies called nucleoli (nfi/kle-6-li). A cell is therefore 

 defined as a mass of protoplasm composed of cytoplasm and 

 nucleus (Figures 2 and 3). 



2. Tissue. — The cells are of many shapes and sizes, 

 and in the bodies of all but microscopic plants and animals 

 are united to help the 

 plant or animal carry 

 on its life processes. 

 This union of cells to 

 do a certain work is 

 called a tissue, and the 

 usual definition is : a 

 tissue is a group of simi- 

 lar cells that do a similar 

 work (Figure 4). 



3. Organs. — In all of 

 the higher animals the 

 tissues are united into 

 skin, arms, stomach, 

 and so on, or in plants into leaf, branch, etc. Such struc- 

 tures are called organs ; an organ is defined as a group of 

 tissues that do a given work in the animal or plant. 



4. The Organ System. — When different organs com- 

 bine to carry on such a general life process as digestion, 

 all of the parts that assist in this process are described as 

 an organ system, as the system of digestive organs (Fig- 

 ures 178 and 179, pages 163 and 165). 



Figure 4. — Tissue. 

 Compare these cells with Figures 2 and 3. 



