' THE BLOOD. 167 



passes through a variety of tissues and is subjected to many differ- 

 ent conditions, its color varies from a scarlet red in the arteries to 

 a bluish red in the veins. It is the presence of the oxygen in com- 

 bination with haemoglobin that gives to the arterial blood its bright 

 color. Lessened oxygen means excess of carbon dioxide, and it is 

 the presence of the latter that gives to venous blood its characteristic 

 bluish-red color. 



When normal blood is drawn from a blood-vessel and placed 

 as a very thin film upon a glass slide, it is found to be opaque, and 

 printed matter cannot be read through it. This opacity is produced 

 by differences of refraction possessed by its several components. 



The healthy red color of the nails, conjunctiva, lips, ears, and 

 mucous membranes in general is due to the presence of the blood. 

 When there is insufficient supply to these parts, temporarily in 

 fainting or for a longer period, as in anaemia, they become pale 

 and waxy in color. In asphyxia and certain heart affections there 

 is a want of proper oxidation, with a resultant bluish color to the 

 above-named parts. 



The color of the blood is changed by the action of poisons. 

 The most marked alteration is the cherry- red color produced by the 

 toxic action of carbon monoxide. This red color is deeper than that 

 of arterial blood. 



Ueaction. The reaction of blood is alkaline. This alkalinity 

 is variable in amount. Thus, it is diminished after great muscular 

 exertion, owing to the formation and presence in it of a large 

 quantity of sarcolactic acid. After long-continued ingestion of 

 soda the alkalinity is increased; after the use of acids it is dimin- 

 ished. In no case, however, does it become distinctly acid. To test 

 the alkalinity of the blood, dry, faintly reddened glazed litmus-paper 

 is used. Upon it is placed a drop of blood, which is allowed to 

 remain for half a minute, to be then wiped off with a weak salt 

 solution. The result is a blue spot upon a red background. 



The alkalinity shown by titration with tartaric acid is the 

 amount of alkali Na, in combination with weak acids, as carbonic or 

 phosphoric, and is known as "titration alkalinity." It has been 

 extensively studied in disease. The average alkalinity of human 

 blood by titration with a standard acid after the corpuscles have 

 been broken up is that of a 0.2- to 0.3-per-cent. solution of sodium 

 hydrate. However, the true alkalinity of the blood is the number 

 of hydroxyl ions (OH"~) free in the solution. Physical chemistry 

 by the electrometric method has shown that blood is a liquid very 



