124 ADVANCED PHYSIOLOGY 



This is a proteid known as fibrinogen which is so completely 

 dissolved as to be quite invisible. 



Red Blood Corpuscles. Red blood corpuscles are minute 

 discs, having a diameter of about ^Vir inch (.007 mm.), and a 

 thickness of yinnnF ' inc ^ (-0025 mm.). There are about 5,000,- 

 000 of these in a drop of blood no larger than the head of an 

 ordinary pin. When freshly drawn from the body these loz- 

 enge-shaped discs appear concave on each side (Fig. 10), 

 but while circulating in the vessels they are frequently cup- 

 shaped. While, in some respects, they are like other cells in 

 the body they have no nucleus and no power of division. 

 This absence of a nucleus is found only in a group of animals 

 known as mammals; i. e. in those which have bodies covered 

 with hair, which suckle their young, and which have distinct 

 thoracic and abdominal cavities separated by a diaphragm. 

 Fishes, frogs, reptiles and birds are not mammals, and their 

 red blood corpuscles are nucleated. 



Each corpuscle consists of two parts: (1) a spongy mass, 

 called the stroma; (2) a red liquid which is held in this stroma 

 somewhat as water is held in a sponge. The red color of the 

 corpuscle is due to this liquid, which is called haemoglobin. 

 It is the millions of these red corpuscles with their hemo- 

 globin that give the red color to the blood. 



Haemoglobin is not always of the same color. If it is 

 mixed with plenty of air, it absorbs a great deal of oxygen, 

 becomes a bright, crimson red, and is then spoken of as 

 oxyhaemoglobin. This is its condition in the blood in the 

 arteries. If the oxygen is withdrawn, however, the haemo- 

 globin assumes a darker color, and is called reduced haemo- 

 globin. This is its condition in the blood in the veins re- 

 turning from the body to the heart. Thus the corpuscles are 

 turned from a dark red to a bright red color as they come in 

 contact with the oxygen of the air in the lungs. This power of 

 absorbing and giving up oxygen is the foundation of respira- 

 tion and is dependent upon the presence of fresh air. Haem- 



