On Blood- Platelets. 



453 



that the average red cell count for the normal rat lies approximately between 

 9,000,000 and 10,000,000 cells per cubic millimetre, the average platelet 

 count lies approximately between 700,000 and 900,000 per cubic millimetre. 

 The variations which have been found- include not only individual differences 

 of different rats, but also differences due to age, feeding, and those due to the 

 personal factor involved in the technique. The various counts made on rats 

 showing complete recovery from the vitamin A deficiency (see fig. 4 and 

 Table) and from the effects of radium (see fig. 5) give similar figures. 



Effect of Vitamin A Deficiency on Platelets. — The rats were kept on a basal 

 ration of casein, starch, autoclaved olive oil and the usual salt-mixture, to 

 which vitamin B was added in the form of marmite. The olive oil prepared 

 as above was known to be free from vitamin A. The casein was freed from 

 the fat-soluble vitamin in some experiments by repeated extraction with 

 alcohol and ether, in others by heating in shallow trays for 24 hours to 130° 

 in air. On such a diet free from the fat-soluble vitamin the platelets show a 

 progressive diminution in their number, and this " thrombopenia " — as it may 

 be called — proceeds pari passu with the decline in the general condition of 

 the animal. Thus in the one extreme condition, when the weight of the 

 animal becomes stationary directly the vitamin A is withheld, and when eye 

 symptoms develop within two months, the fall in the number of platelets is 

 rapid and pronounced. Taking the opposite extreme, when the animal 

 continues to grow at an almost normal rate for several, weeks, and infective 

 conditions do not make their appearance until much later or not at all, then 

 the fall in the number of platelets is delayed and less pronounced, though 

 still distinct. In fact, a slight fall in the number of platelets may sometimes 

 be the only sign of the vitamin A deficiency, the rat looking quite normal 

 and healthy, and having perhaps only a slightly subnormal weight. Our 

 observations show that infective conditions (xerophthalmia, etc.) do not 

 develop until the platelets have fallen below about 300,000 per cubic milli- 

 metre. 



It is important to note that the onset of these infective conditions depends 

 on the level to which the platelets have fallen, and not on the length of time 

 to which the rats have suffered from a vitamin A deficiency, nor on exposure 

 to infection, as will be shown presently. When the deficient vitamin A is 

 again supplied after a low platelet count has been established, the number of 

 platelets increases. Here again there is a close parallelism between the rate 

 of increase in the number of platelets and the degree of improvement in the 

 animal. 



These statements are based on and illustrated by the experimental data 

 given in the following figures and Tables, which explain themselves and need 



