8 THE HUMAN BODY 



so modified to meet the special demands of these tissues as not to 

 be easily recognized. The typical cell (Fig. 5) consists of a mass 

 of living substance, known as protoplasm, of a semi-liquid, gelat- 

 inous consistency, about 0.01 millimeter (2^ 7 inch) in diameter. 

 The protoplasm is usually not perfectly uniform throughout, but 

 shows granules or fine, transparent net works through its sub- 

 stance. Imbedded in the protoplasm is a small structure of dis- 

 tinctly different appearance from the rest of the cell. This is the 

 nucleus. It presents highly characteristic features which will be 

 studied in later paragraphs (p. 23). The mass of cell protoplasm 

 outside the nucleus is called cytoplasm. In general cytoplasm 

 seems to be, as stated above, a relatively simple granular or 

 vesicular mass. As figure 5 indicates, however, there are typic- 

 ally certain definite structures associated with the cytoplasm. 

 The significance of these will be considered later (p. 24). In the 

 highly organized tissues, muscle and nerve, the cytoplasm pre- 

 sents, in addition, complexities of structure suited to the special 

 functions of these tissues. 



Tissues. The individual cells are grouped into masses which 

 are larger or smaller according to the region in which they occur. 

 Obviously only by such a grouping can so large and complex a 

 structure as the body be built of microscopic units. Any cell 

 mass in which the cells are of one type is called a tissue. Thus we 

 speak of muscle tissue or gland tissue according as the cells which 

 make up the tissue in question are muscle cells or gland cells. 

 Many kinds of tissue are widely distributed through the body, 

 others occur only in special parts. The various tissues will be 

 studied in detail in later chapters. 



Chemical Composition of the Body. In addition to the study 

 of the Body as composed of tissues and organs which are optically 

 recognizable, we may consider it as composed of a number of 

 different chemical substances. Ttyis branch of knowledge, which 

 is still very incomplete, really presents two classes of problems. 

 On the one hand, we may limit ourselves to the examination of 

 the chemical substances which exist -in or may be derived from 

 the dead Body, or, if such a thing were possible, from the living 

 Body entirely at rest; such a study is essentially one of structure 

 and may be called Chemical Anatomy. But as long as the Body 

 is alive it is the seat of constant chemical transformations in its 



