THE BLOOD 23 



the fluid condition of the blood in the body of the leech are due 

 to the action of this an ti thrombin. 



Clotting of blood is retarded if the fluid as shed is received into 

 a vessel the wall of which is thinly coated with oil. The shape of 

 the collecting vessel has also an influence over coagulation, 

 clotting being much slower in a deep, smooth vessel than in a 

 rough, shallow one. By increasing the foreign surface to which 

 the blood is exposed clotting is hastened. This is the explanation 

 of the influence of washing a bleeding wound, and the application 

 of compresses excites in some way or other the formation of 

 thrombin. 



The Extractives of the blood are fats, cholesterin, lecithin, 

 kreatine, urea, hippuric acid, uric acid, and grape-sugar, all in 

 small and varying quantities. The amount of fat in the blood 

 during digestion is 0*4 to 06 per cent. ; in dogs fed on a fatty 

 diet it may reach 1*25 per cent., and may give the serum a milky 

 appearance. There is twice as much fat in the serum of recently- 

 fed horses as in the serum of those kept starving. Other 

 extractives such as soaps are found to the extent of 0*05 to 01 per 

 cent. ; urea, 0*02 to 0-04 per cent. ; sugar, o*i to 0-15 per cent. 

 The corpuscles contain neither sugar nor fat, and possess a larger 

 amount of cholesterin and lecithin than the plasma. 



The characteristic Difference between Arterial and Venous 

 Blood is that the former contains more oxygen and less carbonic 

 acid, though there is always, in fully arterialised blood, about 

 twice as much carbon dioxide as there is oxygen (see p. 122). 

 Arterial blood also contains more water, fibrinogen, extractives, 

 salts, and sugar, fewer blood corpuscles, and less urea ; its 

 temperature is, on the average, i° C. lower. The dark colour of 

 venous blood is not due to the greater amount of C0 2 it contains, 

 but to the diminution of oxygen in the red blood-cells. The 

 alteration in colour effected by the addition of reagents and gases 

 to blood is probably due partly to alterations in the shape of the 

 corpuscles themselves, which become more concave on the 

 addition of oxygen and less concave on its removal, and also to 

 the fact that oxyhemoglobin is brighter in colour than reduced 

 haemoglobin. 



The Salts of the blood are divided between the plasma and 

 the corpuscles. The distribution of these is not the same in all 

 animals ; in the horse and pig, for example, sodium only exists 

 in the plasma and none in the corpuscles, while in both animals 

 the potassium in the corpuscles is very high ; in the ox and dog 

 both corpuscles and plasma contain sodium. Sodium chloride 

 is the most abundant salt of the blood, potassium chloride and 

 .sodium carbonate come next, and lastly phosphates of calcium, 

 magnesium, and sodium. The chief inorganic substance of the 



