HISTOLOGY 145 



ganic material as organic in bone. The long bones of the body, 

 such as the femur, have a central marrow cavity filled with bone 

 marrow, where red blood cells are formed. This marrow is, how- 

 ever, not osseous tissue but is merely contained in the cavities of 

 bones. The bone cells (Fig. 79 J and K) He in lacunae within the 

 matrix. A very typical arrangement is that of the Haversian sys- 

 tem. This consists of a central Haversian canal, containing an 

 artery, a vein, and a nerve, surrounded by concentrically arranged 

 rows of lacunae which are in communication with one another and 

 with the central canal by means of minute spaces, the canaliculi 

 (Fig. 79 L). 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 stel- 

 late mesenchyme cells become transformed into rounded cells 

 which serve as storage depots for fat (Fig. 79 F) . In fully developed 

 adipose cells there is a very large drop of neutral fat with only a 

 film of cytoplasm surrounding it and containing the nucleus. The 

 large drop of fat is formed during the specialization of fat-storing 

 cells, by the coalescence of numerous finer drops that are deposited 

 in the cytoplasm. 



Vascular tissue, which is subdivided into hlood and lym-ph, is 

 characteiized by its fluid intercellular material, the plasma. In 

 blood, two kinds of cells are suspended in the pla.sma. Of the.se the 

 red cells, or erythrocytes, contain the iron-bearing haemoglobin, in 

 combination with which o.xygen is carried in the blood (Fig. 80 A 

 and B). In mammals, the red blood cells lose their nuclei at matu- 

 rity, are consequently veiy short-lived, and must be constantly 

 replaced. The chief source of red cells is the bone marrow, but 

 some are apparently formed in the spleen. Of the white blood cells, 

 the leucocytes are granular, irregular in form, and move by changing 

 their shapes, in the same manner as the amoeba, one of the single- 

 celled animals (Fig. 80 C) . Hence, they are said to possess amoeboid 

 movement. In this way they are able to migrate through the walls 

 of the capillaries. Leucocytes are also able to take solid particles, 

 like bacteria and other foreign bodies, into their cytoplasm and so 

 remove them from other tissues. They so function in the case of 

 infections in any part of the body. Lymphocytes are non-granular 

 white blood cells (Fig. 80 D). The source of white blood cells is 

 chiefly the lymph glands and spleen. Blood plasma is believed to 

 be the carrier for all substances transported by the blood, save 



