BLOOD AND H^EMOPOIETIC ORGANS 77 



to which the red cells can be augmented. Normally the 

 corpuscles make up about half the volume of the blood ; 

 if, then, their number were doubled, the blood would 

 become practically solid. For this reason it is difficult 

 to see how clinical estimations of 10,000,000 red cells 

 per cubic millimetre and upwards, such as are sometimes 

 recorded, can be really correct. 



Ample provision has been made, in adult life at any 

 rate, to meet the demand for an increased supply of red 

 corpuscles which any abnormal destruction of them in 

 the blood-stream entails ; for the red marrow of the long 

 bones, which under normal conditions is confined to their 

 extremities, can, if necessary, encroach upon and displace 

 the marrow fat until the whole interior of the bone 

 becomes a manufactory of red corpuscles. The best 

 example of such an extension of the blood-forming 

 territory is seen in pernicious anaemia, in which the 

 whole of the marrow of the long bones becomes red. 

 In young children such an extension is impossible, for 

 the whole of their marrow is red already. Perhaps that 

 is why young children stand loss of blood badly. On 

 the other hand, it would seem that sometimes the red 

 marrow is congenitally deficient or may disappear, in which 

 case a great diminution of red cells in the blood results.* 



The function of the red cells, as we have seen, is 

 essentially a respiratory one. They carry oxygen from 

 the lungs to the tissues, and help in conveying back 

 carbonic acid from the tissues to the lungs. This they 

 are able to do in virtue of the fact that they contain 



* For a report of such a case, see Muir, Brit. Med. Journ., 

 1900, ii. 911. 



