Blood and Lymph. 39 



trated an opaque white appearance similar to that seen in the 

 peripheral nerves, and is hence described as white substance. 

 In the spinal cord (Fig. 19) the grey substance is disposed as a 

 central core, the white substance as a peripheral investment. 

 A similar relation is found in the basal portion of the brain, but the 

 characteristic pattern in the cerebral hemispheres and in the cere- 

 bellum is one in which the grey substance forms a peripheral, 

 investing, or cortical layer. 



5. Blood and Lymph. 



Blood (Fig. 20) is fundamentally a cellular material, but owing 

 to the fact that the cells or corpuscles are contained in a fluid 

 medium, the plasma, it does not take on the features of the ordinary 

 tissues of the body. It is for this reason also that, notwithstanding 

 its important function, the appearances pre- 





sented by the blood in dissection especially of 1^ _. H 



preserved animals are negligible. The cellular / 



materials consist of (a) erythrocytes or red 

 blood cells, microscopic circular discs of fairly 

 definite though not rigid contour, containing 

 in the adult condition no nucleus. They have a corpuscfes'of the rabbit, 

 yellow color, which is . due to the presence of ' ^'^^' ^' '"^° 

 haemoglobin, and which gives to blood its deep red color when seen 

 in bulk. The haemoglobin is the specific carrier of oxygen which it 

 combines chemically. Arterial, oxygenated blood is bright red, 

 while venous blood is dark red or bluish. The number of red blood 

 cells is relatively somewhat greater in the rabbit than in man, 

 there being over six millions contained in each cubic millimetre. 

 The cells are form^ed in the vascular area of the embryo, later in 

 the spleen and liver, and finally in the marrow of bones ; (b) 

 leucocytes and lymphocytes, also termed white or colorless 

 blood cells. They are amoeboid, nucleated cells, present in much 

 smaller numbers in blood, and in lymphatic vessels, and are formed 

 in the lymph glands and spleen. They are capable of passing 

 through the walls of the smaller vessels, and 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- 



