MICROSCOPIC STRUCTURE OF THE BLOOD. 281 



placed. The color of carbon-monoxid hemoglobin is a cherry red, 

 and the blood in persons poisoned with it has this color. 



Nitric-oxid Hemoglobin. Nitric oxid will also displace the 

 oxygen from oxyhemoglobin, and the nitric-oxid hemoglobin which 

 results is a more stable compound than carbon-monoxid hemo- 

 globin. It consists of one molecule of hemoglobin and one of 

 nitric oxid. 



Carbo-hemoglobin. Hemoglobin will also unite with carbon 

 dioxid, and it has been demonstrated that each gas acts with refer- 

 ence to hemoglobin independently of the others i. e., that even 

 when a solution of hemoglobin is nearly saturated with oxygen, it 

 can still take up as much carbon dioxid as when no oxygen is 

 present. It has been suggested that the explanation of this is that 

 the oxygen unites with the portion which gives the color, hemochro- 

 mogen, and the CO 2 with the proteid portion. 



Methemoglobin. This is regarded as being chemically the same 

 as oxyhemoglobin, except that the union of the hemoglobin and 

 oxygen is more stable. Oxyhemoglobin may be converted into 

 methemoglobin by the action of many substances, among which 

 may be mentioned potassium ferricyanid, nitrites, potassium 

 permanganate, etc. It has been demonstrated that when this con- 

 version takes place the whole of its oxygen passes into such inti- 

 mate relationship with the hemoglobin that it cannot be displaced 

 by carbon monoxid. Methemoglobin also crystallizes, the form of 

 the crystals being the same as that of oxyhemoglobin. Methemo- 

 globin is the form in which the blood-coloring matter exists when 

 blood has been for a considerable time exposed to the air. Jt is 

 also known as reduced hematin. 



Hematin. Various formulae have been given to represent this 

 substance. That of Hoppe-Seyler is C^H^^FeOg. When oxy- 

 hemoglobin is decomposed by acids or alkalies in the presence of 

 oxygen hematin results. 



Hemin. This is chemically hematin hydrochlorid, C 34 H^ 5 N 4 - 

 FeO 5 HCl. If to a drop of blood on a glass microscopic slide is 

 added a drop or two of glacial acetic acid and the mixture is boiled, 

 after evaporation there will be found, if examined by the micro- 

 scope, brownish prismatic crystals of hemin. These were discovered 

 'by Teichmann, and are also known by his name (Fig. 151). These 

 crystals are so characteristic that this method of producing them 

 is very much used to determine whether colored spots or stains 

 are blood or not. The explanation of the changes which take 

 place is as follows : The hemoglobin is decomposed by the action 

 of the acetic acid into hematin and a proteid ; and at the same 

 time the sodium chlorid is decomposed, and the hydrochloric acid 

 which is set free unites with the hematin. If this test is used in 

 the case of old blood-stains, from which the sodium chlorid may 



