BLOOD , 525 



is a biconcave circular disc about 7.5 (m in diameter, so that it is 

 shaped like an ordinary double concave lens. Its outer envelope 

 is semipermeable, as is shown by the ease with which it alters 

 its shape in solutions of different strengths. It appears yellow, 

 not red, under the microscope, why is not known, and piles of the 

 cells tend to collect together to form rouleaux, which look like 

 piles of pennies. There are about five million red cells per cubic 

 millimetre in man, rather fewer in woman. In all other vertebrates 

 the red cell is oval and contains a large nucleus, but in mammals 

 this has been lost and the corpuscle seems to contain little or 

 nothing but a colloidal solution of haemoglobin. It is doubtful if 

 it can justifiably be called living. The primitive oval shape of the 

 red cells of lower vertebrates (but not the nucleated condition) is 

 found in the Camelidae alone amongst mammals. 



Less numerous than the red corpuscles are the white cells or 

 leucocytes, of which there are about 7,000 per cubic millimetre. 

 They are nucleated and amoeboid, and are classified by their 

 nuclei and staining reactions into five or six different types. 

 About seventy per cent, of them are polymorphonuclears (Fig. 

 410), 10 /A or a little more in diameter, and with a large lobulated 

 nucleus. They are highly phagocytic and are specially numerous 

 in places where there is bacterial infection. 



Blood platelets, or thrombocytes, are small particles of cyto- 

 plasm, 2 A6 in diameter, of which there are about 200,000 per cubic 

 millimetre. Their function is doubtful. 



LYMPH 



Plasma and white corpuscles are able to escape from the finest 

 blood vessels into the surrounding connective tissue, a process 

 known as diapedesis, and it is presumably from this source that 

 the ' tissue fluid ' of the body is derived. In a healthy body, 

 however, it is so difficult to detect in areolar tissue that many 

 people have denied its existence. In pathological conditions, 

 such as blisters and dropsy, it quickly accumulates, and on 

 theoretical grounds is almost certainly always present. The 

 passage of leucocytes through the walls of capillaries, where they 

 have been filmed forcing their way between the cells, has been 

 observed, and this supports the view that the histiocytes of con- 

 nective tissue are simply white blood corpuscles. It is obvious that 

 this leakage from the blood vascular tissue could not go on 



