BLOOD AND LYMPH 33 



negligible, in spite of the importance of its functions. The cellular 

 components comprise (a) erythrocytes, (b) leucocytes, and {c) 

 platelets. The erythrocytes or red blood cells are microscopic 

 biconcave, circular discs of uniform size and definite though not 

 rigid contour, containing no nuclei in adult mammals. They are 

 so soft and flexible that they squeeze readily through capillaries of 

 diameter smaller than their own. They have a yellow colour when 

 seen singly, or deep red when observed in bulk, on account of the 

 presence of haemoglobin. The latter material is 

 the specific carrier of oxygen, with which it 

 forms a readily dissociated chemical compound. 

 Arterial, oxygenated blood is bright red, while 

 venous blood is dark red. The number of red 

 blood cells is relatively somewhat greater in the ^^^ ^g ^^^ ^^^^^ 

 rabbit than in man, there being over six millions f^fP^efp' pJoSe/^'"'''^' 

 contained in each cubic millimetre. The cells 

 are formed first in the yolk-sac wall of the embryo, later in the 

 spleen and liver, and in the marrow of bones. The leucocytes, also 

 termed white or colourless blood cells, are amoeboid, nucleate cells, 

 present in the blood in much smaller numbers than the erythrocytes, 

 but occurring also in lymph. A number of different kinds is recog- 

 nized and named. They are classified first as granular or agranular, 

 the former comprising neutrophiles, eosinophiles, and basophiles, 

 the latter comprising lymphocytes, monocytes, and some others ap- 

 parently transitional. They are formed in the lymph glands, in 

 the spleen, and elsewhere (cf. p. 121). Being capable of passing 

 through the walls of the smaller vessels, they occur more or less 

 throughout the tissues, where they have the function of carrying 

 materials or of ridding the body of injurious substances and bac- 

 teria. The platelets are minute, non-nucleate masses of cytoplasm 

 derived chiefly by fragmentation from cells in the bone marrow. 

 They are related to coagulation of the blood and help to seal up 

 small openings in the walls of the vessels. 



In all multicellular animals, large proportions of the tissues are 

 necessarily more or less distant from the surfaces of absorption and 

 excretion. Blood and lymph, circulating through the vascular 

 system, are the media by which communication with these is kept 

 up, providing for the transportation of materials essential for the 



