234 TEXT-BOOK OF PHYSIOLOGY. 



though there is some experimental evidence for the belief that it is produced 

 in the liver though out of what has not been determined. Beyond the fact 

 that it contributes to the occasional formation of fibrin there is no positive 

 knowledge either as to its origin, its nutritive value, or its final disposition in 

 the blood under normal conditions. 



Fat. The plasma as well as the serum contains a very small quantity 

 of fat in the form of microscopic globules. Though the percentage is nor- 

 mally not above 0.25, yet just after a meal rich in fatty matter the amount 

 may be so great as to give to the blood a milky or opalescent appearance. 

 Within a few hours, however, this excess of fat disappears from the blood, 

 though its immediate disposition is unknown. Soaps or alkaline salts of the 

 fat acids, though formed during the digestion of fats, are not present in 

 the blood. Lecithin and cholesterin are present in very small quantities. 



Sugar. Sugar is present in the blood in the form of dextrose, and is 

 now regarded as a normal constituent. The amount is about i part per 

 thousand, though it may be present to the extent of 15 parts per thousand. 

 Beyond this, the excess soon appears in the urine. 



Extractives. The blood contains a series of nitrogenized bodies, such 

 as urea, uric acid, creatin, creatinin, xanthin, etc., which result from the 

 katabolic changes in nerve- and muscle-tissues as well as from subsequent 

 chemic combinations and decompositions. Though constantly absorbed 

 from the tissues, they seldom accumulate beyond a small amount, since 

 they are constantly being eliminated from the blood by the various ex- 

 cretory organs. 



Inorganic Salts. The inorganic salts of the plasma are chiefly sodium 

 and potassium chlorids, sulphates, and phosphates, together with calcium 

 and magnesium phosphates. Of the salts, sodium chlorid is the most 

 abundant, amounting to 0.56 parts per hundred. Calcium phosphate is 

 present in small quantity 2 parts per 1000. This salt is not present to 

 the same extent in serum for the reason that it became a constituent of 

 fibrin at the time of coagulation. In other respects serum differs but 

 slightly from plasma in the proportions of its saline constituents. 



HISTOLOGY OF THE RED CORPUSCLES OR ERYTHROCYTES. 



The histologic features of the red corpuscles are readily observed in a 

 drop of freshly drawn blood when examined microscopically. The field of 

 the microscope will be seen to be crowded with red corpuscles floating in a 

 clear transparent fluid the plasma. Here and there will also be seen a white 

 corpuscle, round or irregular in shape, and granular in appearance. Within 

 a short time a characteristic phenomenon takes place: viz., the arranging of 

 the corpuscles in the form of columns of varying lengths, resembling rolls of 

 coins. These rolls interlace with each other at all angles and form a network 

 in the meshes of which lie individual red and white corpuscles. (See Fig. 

 97.) The cause of this tendency of the corpuscles to adhere to one another 

 is not definitely known. Since it does not take place in circulating blood, and 

 since it is to a great extent prevented by defibrinating the blood, it has been 

 supposed to be dependent on the formation of some adhesive substance con- 

 nected with the formation of fibrin. 



