THE RESPIRATORY SYSTEM. 389 



of color to brownish red, after which, there are no longer any 

 reducing effects, it would seem that the oxygen-carrying power 

 and iron are associated. This iron-free haematin is named 

 hoematorporphyrin or hcematoin. 



Hcemin is hydrochlorate of haematin (Teichmann's crystals), 

 and may be formed by adding glacial acetic acid and common 

 salt to blood, dried blood-clot, etc., and heating to boiling. This 

 is one of the best tests for blood, valuable in medico-legal and 

 other cases. < 



When oxy-hsemoglobin stands exposed to the air, or when 

 diffused in urine, it changes color and becomes, in fact, another 

 substance — methcBmoglobin, irreducible by other gases (CO, etc.), 

 and not surrendering its oxygen in vacuo, though giving it up 

 to ammonium sulphide, becoming again oxy-haemoglobin, when 

 shaken up with atmospheric air. Its spectrum differs from 

 that of oxy-hsemoglobin in that it has a band in the red end of 

 the spectrum between the C and I) lines. Hmmatoidin is some- 

 times found in the body as a remnant of old blood-clots. It is 

 probably closely allied to if not identical with the bilirubin 

 of bile. 



Comparative. — While hsemoglobin is the respiratory agent in 

 all the groups of vertebrates, this is not true of the inverte- 

 brates. Red blood-cells have as yet been found in but a few 

 species, though haemoglobin does exist in the blood plasma of 

 several groups, to one of which the earth-worm and several 

 other annelids belong. It is interesting to note that the respir- 

 atory compound in certain families of crustaceans, as the com- 

 mon crab, horseshoe-crab (limulus), etc., is blue, and that in 

 this substance copper seems to take the place of iron. 



The Nitrogen and the Carbon Dioxide of the Blood. -The 

 little nitrogen which is found in about equal quantity in venous 

 and arterial blood seems to be simply dissolved. The relations 

 of carbonic anhydride are much more complex and obscure. 

 The main facts known are that — 1. The quantity of this gas is 

 as great in serum as in blood, or, at all events, the quantity in 

 serum is very large. 2. The greater part may be extracted by 

 an exhaustion-pump ; but a small percentage (2 to 5 volumes 

 per cent) does not yield to this method, but is given off when 

 an acid is added to the serum. 3. If the entire blood be sub- 

 jected to a vacuum, the whole of the CO2 is given off. 



From these facts it has been concluded that the greater part 

 of the CO2, exists in the plasma, associated probably with sodium 



