52 BLOOD. 



guish arterial from venous blood, and which may be given off from blood 

 when exposed to an atmosphere, according to the composition of that 

 atmosphere. These gases of blood we shall study in connection with res- 

 piration. 



The normal blood consists of corpuscles and plasma. 



If the corpuscles be supposed to retain the amount of water proper to 

 them, blood may, in general terms, be considered as consisting by weight 

 of from about one-third to somewhat less than one-half of corpuscles, the 

 rest being plasma. As we have already seen, the number of corpuscles in 

 a specimen of blood is found to vary considerably, not only in different ani- 

 mals and in different individuals, but in the same individual at different 

 times. 



The plasma is resolved by the clotting of the blood into serum and fibrin. 



35. The serum contains in 100 parts : 



Proteid substances , . about 8 or 9 parts. 



Fats, various extractives, and saline matters " 2 or 1 part. 



Water ...,,.. 90 parts. 



The proteids are paraglobulin and serum-albumin (there being probably 

 more than one kind of serum-albumin) in varying proportion. We may, 

 perhaps, roughly speaking, say that they occur in about equal quantities. 



Conspicuous and striking as are the results of clotting, massive as appears 

 to be the clot which is formed, it must be remembered that by far the greater 

 part of the clot consists of corpuscles. The amount by weight of fibrin re- 

 quired to bind together a number of corpuscles, in order to form even a 

 large, firm clot, is exceedingly small. Thus, the average quantity by weight 

 of fibrin in human blood is said to be 0.2 per cent. ; the amount, however, 

 which can be obtained from a given quantity of plasma varies extremely, 

 the variation being due not only to circumstances affecting the blood, but to 

 the method employed. 



The fats, which are scanty, except after a meal or in certain pathological 

 conditions, consist of the neutral fats stearin, palmitin, and olein with a 

 certain quantity of their respective alkaline soaps. The peculiar complex 

 fat lecithin occurs in very small quantities only ; the amount present of the 

 peculiar alcohol cholesterin, which had so fatty an appearance is also small. 

 Among the extractives present in serum may be put down nearly all the 

 nitrogenous and other substances which form the extractives of the body 

 and of food, such as urea, kreatin, sugar, lactic acid, etc. A very large 

 number of these have been discovered in the blood under various circum- 

 stances, the consideration of which must be left for the present. The pecu- 

 liar odor of blood or of serum is probably due to the presence of volatile 

 bodies of the fatty acid series. The faint yellow color of serum is due to a 

 special yellow pigment. The most characteristic and important chemical 

 feature of the saline constitution of the serum is the preponderance, at 

 least in man and most animals, of sodium salts over those of potassium. 

 In this respect the serum offers a marked contrast to the corpuscles. Less 

 marked, but still striking, is the abundance of chlorides and the poverty 

 of phosphates in the serum, as compared with the corpuscles. The salts 

 may in fact briefly be described as consisting chiefly of sodium chloride, 

 with some amount of sodium carbonate, or more correctly sodium bicar- 

 bonate, and potassium chloride with small quantities of sodium sulphate, 

 sodium phosphate,, calcium phosphate, and magnesium phosphate. And 

 of even the small quantity of phosphates found in the ash, part of the 

 phosphorus exists in the serum itself, not as a phosphate, but as phos- 

 phorus in some organic body. 



