146 



CELLS OF THE VERTEBRATE BODY 



oxygen. It possesses the capacity of clotting when drawn from 

 the vessels. During clotting, a mass of fine fibers, composed 

 of a simple protein material, the fibrin, appears, and the cells are 

 held in its meshes. The plasma that is left can be separated from 

 fibrin and cells and forms the serum that is used in immunizing 

 against certain diseases. Lymph differs from blood in that it does 

 not contain erythrocytes but is composed of plasma and colorless 

 cells. The importance of lymph as the pathway between blood 

 and tissues should be recalled. 



Contractile Tissue. — Contractile tissues are known as muscles, 

 and are of three kinds: non-striated, cardiac, and striated. The 



%a 



A 



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D -^ c ""--" " C ^ 



Fig. 80. — A. Mammalian erythrocytes arranged in rouleau.x. B. Single 

 mammalian erythrocyte. C. Leucocytes with polymorphic nuclei. D. Lym- 

 phocytes. 



cytoplasm of muscle cells is characterized by the presence of 

 numerous fine fibers which are longitudinally placed. It is the 

 shortening and thickening of these muscle fibrilloe that produces 

 contraction of the individual cell and is therefore responsible for 

 the particular function of this kind of tissue, the production of 

 motion. Non-striated muscle cells are typically spindle-shaped 

 with the nucleus centrally placed (Fig. 81 A and B). These cells 

 usually occur in sheets, which are formed by many of the cells lying 

 side by side and held together loosely by fibrous connective tissue. 

 This kind of muscle is sometimes called involuntary muscle, because 

 it is found in the wall of the digestive tract, in the urinary bladder, 

 and in blood vessels, where it is not under conscious nervous con- 

 trol. Cardiac muscle is found only in the heart and is capable of 

 rhythmical contractions (Fig. 81 F). The cells are arranged in the 



