Chap. 7 



TISSUES 



121 



Fig. 7.14. Skeletal or striated muscle cells. A and B, in long section; C and D, 

 in cross section. Note the nuclei with large nucleoli. The differences in appear- 

 ance are due to different methods of preparation, an example of what often 

 happens to preserved material. (Courtesy, Nonidez and Windle: Textbook of 

 Histology, ed. 2. New York, McGraw-Hill Book Co., Inc. 1953.) 



Red and White Muscle. The cells of dark red muscle (dark meat) 

 contain an extra amount of muscle hemoglobin (myoglobin), and abundant 

 cytoplasm. This muscle also has a large blood supply and is usually active 

 for long periods of time. Pale muscle fibers (white meat) contain less cyto- 

 plasm, less myoglobin, and have a smaller blood supply. The color of muscle 

 also varies with the animal; in birds, red and white; in rabbits, red and white; 

 in nearly all human muscles, a mixture of both types. 



Cardiac Muscle. In all vertebrates the heart is composed of a network 

 of striated muscle fibers. They are unique in being branched and having 

 centrally placed nuclei and intercalated, or literally, inserted discs, that is, 

 dark bands that cross the fibers at irregular intervals whose function is not 

 known (Fig. 7.15). 



Nervous Tissue 



The functioning of nervous tissue is due to two properties of protoplasm: 

 irritability, the power to react to various chemical and physical stimuli, and 



