22 A MANUAL OF PHYSIOLOGY 



conditions. But there is no good foundation for the statement 

 sometimes rather fancifully made that the spleen is in any special 

 sense the ' graveyard of the red corpuscles.' Some of the coloured 

 corpuscles may break up in the blood itself, forming granules of 

 pigment, which may then be taken up by the liver, spleen, and 

 lymph glands. Indeed, it is probable that a large proportion of 

 the worn-out erythrocytes are finally destroyed in the blood- 

 stream. The portal circulation may be more than other vascular 

 tracts a seat of this natural decay, perhaps in virtue of the 

 presence of substances with a haemolytic action (p. 27) absorbed 

 from the alimentary canal. 



The lymphocytes are undoubtedly derived from the lymph. 

 They are identical with the small lymph-corpuscles, and have 

 little, if any, power of amoeboid movement. They are formed 

 largely in the lymphatic glands, for the lymph coming to the 

 glands is much poorer in corpuscles than that which leaves them. 

 The lymphatic glands, however, are not the only seat of forma- 

 tion of lymphocytes, for lymph contains some corpuscles before 

 it has passed through any gland ; and although a certain number 

 of these may have found their way by diapedesis from the blood, 

 others are formed in the diffuse adenoid tissue, or in special col- 

 lections of it, such as the thymus, the tonsils, the Peyer's patches 

 and solitary follicles of the intestine, and the splenic corpuscles. 

 The hyaline cells are possibly developed by the enlargement of 

 lymphocytes. It is probable that the eosinophile cells, the poly- 

 morphonuclear neutrophiles, and the ' mast ' cells are formed in 

 the bone-marrow. To a very small extent white blood-corpuscles 

 may multiply by karyokinesis or indirect division in the blood. 



The fate of the leucocytes is even less known than that of 

 the red corpuscles, for they contain no characteristic substance, 

 like the blood-pigment, by which their destruction may be traced. 

 That they are constantly disappearing is certain, for they are 

 constantly being produced. Not a few of them actually escape 

 from the mucous membranes of the respiratory, digestive, and 

 urinary tracts. The remnants of broken-down leucocytes have 

 been found in the spleen and lymph glands. It must be assumed 

 that many break up in the blood-plasma itself. 



Physical and Chemical Properties of the Blood. 



Fresh blood varies in colour, from scarlet in the arteries to 

 purple-red in the veins. It is a somewhat viscid liquid, with a 

 saline taste, and a peculiar odour. 



Viscosity of Blood. The viscosity of normal dog's blood is 

 about six times greater than that of distilled water at body 

 temperature. It can be determined by allowing the blood to 



