I. — PHYSIOLOGY. 199 



Types of Anoxceviia. 



And now to pass to the consideration of the various types of 

 anoxaemia. 



Anoxcemia is by derivation want of oxygen in the blood. 



Suppose you allow your mind to pass to some much more homely 

 substance than oxygen — such, for instance, as milk — and consider the 

 causes which may conspire to deprive your family of milk, three 

 obvious sources of milk deficiency will occur to you at once : 



(1) There is not enough milk at the dairy ; 



(2) The milk is watered or otherwise adulterated so that the fluid 

 on sale is not really all milk; and 



(3) The milkman from that particular dairy does not come down your 

 road. 



These three sources of milk deficiency are typical of the types of 

 oxygen deficiency. 



The first is insufficient oxygen dispensed to the blood by the lungs. 

 An example of this type of anoxaemia is mountain-sickness. The 

 characteristic of it is insufficient pressure of oxygen in the blood. In 

 mountain-sickness the insufficiency of pressure in the blood is due to 

 insufficient pressure in the air, for, according to our view at all events, 

 the pressure in the blood will always be less than that in air. But 

 this type of anoxaemia may be due to other causes. The sufferer may 

 be in a normal atmosphere, yet for one reason or another the air may 

 not have access to his whole lung. In such cases, either caused by 

 obstruction, by shallow respiration, or by the presence of fluid in the 

 alveoli, the blood leaving the affected areas will contain considerable 

 quantities of reduced haemoglobin. This will mix with blood from un- 

 affected areas which is about 95 per cent, saturated. The oxygen will 

 then be shared round equally among the corpuscles of the mixed blood, 

 and if the resultant is only 85-90 per cent, saturated the pressure of 

 oxygen will only be about half the normal, and, as I said, deficiency of 

 oxygen pressure is the characteristic of this type of anoxaemia. 



The second type involves no want of oxygen pressure in the arterial 

 blood ; it is comparable to the watered milk : the deficiency is really in 

 the quality of the blood and not in the quantity of oxygen to which the 

 blood has access. The most obvious example is anaemia, in which 

 from one cause or other the blood contains too low a percentage of 

 Esemoglobin, and because there is too little haemoglobin to carry the 

 oxygen too little oxygen is carried. Anaemia is, however, only one 

 example which might be given of this type of anoxaemia. There may 

 be sufficient haemoglobin in the blood, but the haemoglobin may be 

 useless for the purpose of oxygen transport ; it may be turned in part 

 into methaemoglobin, as in several diseases, e.g. among workers in the 

 manufacture of some chemicals, and in some forms of dysentery con- 

 tracted in tropical climates, or it may be monopolised by carbon 

 monoxide, as in mine-air. 



Thirdly, the blood may have access to sufficient oxygen and may 

 contain sufficient functional hasmoglobin, but owing to transport trouble 

 it may not be circulated in sufficient quantities to the tissues. The 

 quantity of oxygen which reaches the tissue in unit time is too smaU, 



