SPECIALIZATION OF HUMAN TISSUES 



89 



Compare this magnified drop of frog's blood 

 with the human blood. This is not enlarged as 

 many diameters as the preceding photograph. 

 Notice the oval shape of the red corpuscles and 

 the nucleus in each one. 



highly specialized for secretion, absorption, and protection ; 

 muscle tissue, for contraction ; connective tissue, for binding 

 together various parts of 



the body and for support; 

 nerve tissue, for transmitting 

 stimuli ; and blood tissue, for 

 circulating materials. In gen- 

 eral, the cells of all tissues 

 can perform all the cell func- 

 tions that are necessary for 

 the life of the cell. When 

 cells are highly specialized 

 and are not in direct con- 

 tact with the outside world, 

 some of their functions be- 

 come reduced and are practi- 

 cally lost. Then these cells 

 become dependent upon each other to such an extent that life 

 is impossible without this interdependence. 



The tissue cells of higher animals do not have to seek food. 

 They are supplied with food by the blood. Certain cells store 

 small quantities of animal starch or glycogen and oil. These par- 

 ticles may be digested by the cells when the need arises. Assimi- 

 lation and growth remain as functions of all cells. Cells obtain 

 oxygen from the blood, oxidize the food for the release of energy, 

 and use this energy for their work. The muscle cells and white 

 corpuscles release energy, in the form of mechanical energy, par- 

 ticularly for motion. The epithelial cells use chemical energy, and 

 nerve cells use nervous energ3\ The connective tissues and red 

 corpuscles need only sufficient energy to perform their general cell 

 functions. Most of the tissue cells lose their power to divide mi- 

 totically. The epithelial cells and the white corpuscles are the 

 only ones, thus far studied, which retain this power throughout life. 



WH. FITZ. AD. BIO. — 7 



