78 APPLIED PHYSIOLOGY 



haemoglobin, to a consideration of which remarkable 

 substance we must now turn. 



Haemoglobin is in many ways a unique compound. 

 It has the honour of possessing the largest molecule in 

 the body, and of being the only body protein which can 

 be easily obtained in a crystalline form. It consists of 

 an iron-containing pigment (haematin) united to a histon 

 termed * globin.' It is to the fact that it contains iron 

 that it owes its power of taking up oxygen ; the protein 

 part of the compound merely serves the humble function 

 of acting as a sort of lifebuoy to the heavy iron-containing 

 part of the molecule, and floating it along in the blood- 

 stream. Oxygen is, unfortunately, not the only gas 

 with which haemoglobin is ready to enter into partner- 

 ship. It has an inconvenient affinity for carbonic oxide, 

 nitric oxide, and other gases, and when it is united to 

 them its proper respiratory function is at an end. Hence 

 a patient poisoned by carbonic oxide really dies from 

 sheer inability to get oxygen conveyed from his lungs 

 to his tissues. Oxygen inhalation, which increases the 

 amount of oxygen dissolved in the plasma as opposed to 

 that in the corpuscles, may tide over the difficulty until 

 the gradual dissolution of the unnatural partnership has 

 taken place. 



That haemoglobin is formed like the corpuscles in the 

 red marrow is probable, but not proven. It seems to be 

 deposited in the cells whilst they are still in their 

 nucleated condition, but by what intricate chemical pro- 

 cesses its complex molecule is built up is still a secret. 

 This, however, we do know that haemoglobin is only 

 built up rather slowly, and that it takes some time, after 



