VI. BLOOD AND LYMPH. 



BLOOD. 



A. GENERAL, PROPERTIES: PHYSIOLOGY OP THE CORPUSCLES. 



THE blood of the body is contained in a practically closed system of tubes, 

 the blood-vessels, within which it is kept circulating by the force of the heart- 

 beat. The blood is usually spoken of as the nutritive liquid of the body, but 

 its functions may be stated more explicitly, although still in quite general 

 terms, by saying that it carries to the tissues food-stuifs after they have been 

 properly prepared by the digestive organs; that it transports to the tissues 

 oxygen absorbed from the air in the lungs ; that it carries off from the tissues 

 various waste products formed in the processes of disassimilation, such as 

 urea, uric acid, water, CO 2 , etc. ; and that in warm-blooded animals it aids in 

 equalizing the temperature of the body. It is quite obvious, from these 

 statements, that a complete consideration of the physiological relations of the 

 blood would involve substantially a treatment of the whole subject of physi- 

 ology. It is proposed, therefore, in this section to treat the blood in a re- 

 stricted way to consider it, in fact, as a tissue in itself, and to study its com- 

 position and properties without especial reference to its nutritive relationship 

 to other parts of the body. 



Histological Structure. The blood is composed of a liquid part, the 

 plasma, in which float a vast number of microscopic bodies, the blood-corpus- 

 cles. There are at least three different kinds of corpuscles, known respectively 

 as the red corpuscles; the white corpuscles or leucocytes, of which in turn 

 there are a number of different kinds ; and the blood-plates. As the details 

 of structure, size, and number of these corpuscles belong properly to text- 

 books on histology, they will be mentioned only incidentally in this section 

 when treating of the physiological properties of the corpuscles. Blood-plasma, 

 when obtained free from corpuscles, is perfectly colorless in thin layers for 

 example, in microscopic preparations ; when seen in large quantities it shows a 

 slightly yellowish tint, the depth of color varying with different animals. This 

 color is due to the presence in small quantities of a special pigment, the nature 

 of which is not definitely known. The red color of blood is not due, there- 

 fore, to coloration of the blood-plasma, but is caused by the mass of red cor- 

 puscles held in suspension in this liquid. The proportion by bulk of plasma 

 to corpuscles is usually given, roughly, as two to one. 



Blood-serum and Defibrinated Blood. In connection with the explanation 

 of the term " blood-plasma " just given, it will be convenient to define briefly 



331 



