THE BLOOD. 197 



DARE'S H/EMOGLOBINOMETER. 



Dare's instrument is on the same principle as Von FleischPs, 

 except it compares a film of undiluted blood with a wedge of colored 

 glass. 



The essential parts of the instrument are an automatic pipette 

 for collecting the blood and a graduated color comparison to measure 

 the percentage of haemoglobin contained therein. In making an 

 observation, the instrument is held very much as a field glass, that 

 is, up before the eyes, which are screened from the candle light by 

 it. If the reading is 100, the haemoglobin is normal; if the reading 

 is 70, there is 70 per cent, of the normal. 



To use the instrument, a puncture is made in the lobe of the 

 ear; the first drop of blood is wiped away, whilst the second, which 

 should correspond in size to a drop of water, is touched as it emerges 

 from the wound to either of the three free edges of the pipette, held 

 horizontally, which fills automatically to its margin by capillary 

 attraction. When filled, the excess upon the edge presented to the 

 blood drop is removed by quickly wiping it as the pipette is withdrawn. 



PHYSICAL PROPERTIES OF THE PLASMA. 



Plasma is the fluid part of the blood as it occurs in a healthy 

 condition within the circulatory system. However, upon its removal 

 from the body there is formed in it a solid substance, called fibrin, 

 from elements which it previously held in solution. The fluid 

 which surrounds the clot is termed serum:, it is plasma minus fibrin. 

 Plasma is described as a clear, somewhat viscid fluid; that of man, 

 when strata are examined, is colorless; when in bulk it is slightly 

 yellow because of the presence of a pigment. 



CHEMICAL PROPERTIES OF PLASMA AND SERUM. 



In order to examine plasma, a very great amount of caution is 

 necessary to prevent its coagulation, even after separating the cor- 

 puscles. The most common methods for obtaining it in a liquid 

 state are by the use of the "living test-tube" an excised piece of 

 jugular of a horse filled with blood, and cold as an environment. It 

 has been found that serum differs from plasma only in respect to 

 certain porteids, and, as it is so much easier to handle the serum, 

 the latter is principally used for experimentation. 



Chemically the plasma is composed of inorganic and organic 

 substances, with certain gases. 



