METABOLISM IN VERTEBRATES 



A very typical arrangement is that of the Haversian system. Fhis consists of a 

 central Haversian canal which contains an artery, a vein, and a nerve, sur- 

 rounded by concentrically arranged rows of lacunae in communication with 

 one another and with the central canal by means of minute spaces, the 

 canaliculi (Fig. 3.15L). Lymph circulates in these canaliculi and furnishes 

 a passageway for foods and wastes between blood and cells. 



In adipose tissue there is no intercellular material, and the stellate mesen- 

 chyme cells become transformed into rounded cells which serve to store fat 

 (Fig. 3.\SP^. In fully differentiated adipose cells a large drop of fat is sur- 

 rounded by a film of cytoplasm, which contains the nucleus. The large drop 

 of fat is formed by the coalescence of numerous finer drops that are deposited 

 in the cytoplasm during the specialization of fat-storing cells. Adipose tissue 

 is widely distributed in the body. 



Vascular Tissue. The vascular or circulating tissues are the blood and 

 lymph and are characterized by a liquid intercellular material, the plasma. 

 In blood, two kinds of cells are suspended in the plasma. Of these the red 

 blood cells, or erythrocytes, contain the iron-bearing hemoglobin in combina- 

 tion with which oxygen is carried in the blood (Fig. 3.16^ and B). The 

 average human being has about 6 liters of blood, and each cubic millimeter of 

 it contains 4^2 to 5 million red blood cells. In mammals the red cells lose 

 their nuclei during their differentiation, live only about 125 days, and must be 

 replaced constantly. Red blood cells are differentiated in the red bone 

 marrow in adults, and, if they are not formed in adequate numbers, the indi- 

 vidual becomes anemic. The red cells are destroyed by macrophages located 

 along the walls of the capillaries of the spleen and liver; 7 to 10 million red 

 cells are destroyed every second. 



White blood cells are frequently irregular in shape, since they are capable 

 of amoeboid movement and migrate through the walls of capillaries and 

 among the cells of other tissues (p. 54). Wandering white blood cells can 



fig. 3.16. Vascular tissue. A, mammalian erythrocytes arranged in rouleaux. B, single 

 mammalian erythrocyte (non-nucleated). C, granular leucocytes with polymorphic nuclei; one 

 with irregular cytosome. D, non-granular leucocytes or lymphocytes. 



63 



