388 COMPARATIVE PHYSIOLOGY. 



If, now, to a specimen showing the two bands of oxy-haenio- 

 globin distinctly a few drops of ammonium sulphide or other 

 reducing agent be added, a change in the color of the solution 

 will result, and the single hazy band characteristic of haemo- 

 globin will appear. 



It is not to be supposed, however, that venous blood gives 

 this spectrum. Even after asphyxia it will be difficult to see 

 this band, for usually some of the oxy-hsemoglobin remains 

 reduced ; but it is worthy of note, as showing that the appear- 

 ances are normal, that the blood, viewed through thin tissues 

 when actually circulating, whether arterial or venous, gives 

 the spectrum of oxy-hsemoglobin. At the same time there can 

 be no doubt that the changes in color which the blood under- 

 goes in passing through the capillaries is due chieily to loss of 

 oxygen, as evidenced by the experiments before referred to ; and 

 the reason that the two bands are always to be seen in venous 

 blood is simply that enough oxy-hsemoglobin remains to give 

 the two-band spectrum which prevails over that of (reduced) 

 haemoglobin. We are thus led by many paths to the important 

 conclusion that the red corpuscles are oxygen-carriers, and, 

 though this may not be and probably is not their only func- 

 tion, it is without doubt their principal one. Of their oxygen 

 they are being constantly relieved by the tissues; hence the 

 necessity of a circulation of the blood from a respiratory point 

 of view. 



There are other gases that can replace oxygen and form 

 compounds with haemoglobin ; hence we have CO-hEemoglobin 

 and NO-haemoglobin, which in turn are replaced by oxygen with 

 no little difficulty — a fact which explains why carbonic oxide is 

 so fatal when respired, and, as it is a constituent of illuminat- 

 ing gas, the cause of the death of those inhaling the latter is 

 often not far to seek. Blood may, in fact, be saturated with 

 carbonic oxide by allowing illuminating gas to pass through it, 

 when a change of color to a cherry red may be observed, and 

 which will remain in spite of prolonged shaking up with air or 

 attempts at reduction with the usual reagents. Haemoglobin 

 may be resolved into a proteid (globin) not well understood, 

 and hmmatin. This happens when the blood is boiled (perhaps 

 also in certain cases of lightning-stroke), and when strong acids 

 are added. Haematin is soluble in dilute acids and alkalies, and 

 has then characteristic spectra. Alkaline haematin may be re- 

 duced ; and, as the iron can be separated, resulting in a change 



