30 • THE BLOOD. 



Serum. — This is a thin and usually transparent liquid, of a pale 

 yellowish hue ; it is, however, sometimes turbid, or milky, and this 

 turbidity may depend upon different conditions, but most commonly on 

 excess of fatty particles. The specific gravity of serum ranges from 

 1"025 to 1"030, but is most commonly between 1*027 and 1-028 (Nasse), 

 and is more constant than that of the blood. The solid contents of 

 the serum are not more than 8 or 9 in 100 parts ; the proportion of 

 water being, for males 90*88, and for females 91'71. It is always more 

 or less alkaline. When heated, it coagulates, in consequence of the 

 large quantity of albumin it contains ; and after separation of the 

 albumin, a thin saline liquid remains, sometimes named "serosity." 

 The following ingredients are found in the serum : — 



Albumin. — This principle is partly combined with soda as an 

 albuminate ; its quantity may be determined (after previous removal of 

 paraglobulin) by precipitating it in the solid form by means of heat or 

 alcohol, washing with distilled water, drying, and weighing the mass. 

 Its proportion is about 80 in 1000 of serum, or nearly 40 in 1000 of 

 blood. Serum-albumin differs from albumin obtained ft-om white of 

 egg in the fact of its not being precipitated by ether ; in other respects 

 it closely resembles that substance. Albumin is coagulated and trans- 

 formed into an insoluble variety by heat. 



Albumin is closely allied to iDaraglobulin, fibrinogen, myosin, and many other 

 nitrogenized substances met with in the animal economy : they are therefore com- 

 monly grouped together as alhuminold substances, protein bodies, or jiroteids. 

 Albuminoids are characterised by their low diffusibility and their readiness, when 

 in solution, to take on the solid condition, or to coagulate. They therefore belong 

 to the colloid substances of Graham. They are preciisitated from their solutions 

 by alcohol, mineral acids, tannic acid, corrosive sublimate, and many other 

 metallic salts. They are all coloui-ed yellow by nitric acid, becoming red on 

 subsequent addition of ammonia. Acid nitrate of merciuy produces a red colour, 

 and sulphate of copper and potash a violet colom- in their solutions. The albu- 

 minoids all consist of carbon, hydi'Ogen, oxygen, and nitrogen, together with a 

 small amount of sulphur-. 



By the actioA of the gastric juice ordinaiy albuminoids are transfonued 

 into an exceedingly soluble, diffusible vai-iety termeik 2)rpton. 



Albtmiin combines with both acids and alkalies forming respectively acid- 

 albmnin or syntonin. and alkali-albumin or casein. 



Paraglobulin. — "When serum is diluted with about ten times its 

 bulk of distilled water, and subjected to a stream of carbonic acid, the 

 liquid becomes turbid, and paraglobulin is precipitated. It may also be 

 oljtained from the diluted serum by the cautious addition of acetic acid, 

 but the least excess of acid will re-dissolve the precipitate. Para- 

 globulin is a protein compound, agreeing very nearly with albumin in 

 elementary composition, so far as this has been ascertained. Para- 

 globulin is nearly insoluble in pure water, but readily dissolves on a 

 very slight addition of either an alkali or an acid. Weak acids throw 

 it down from its solution in alkali, but when added in slight excess 

 re-dissolve it. In like manner it is precipitated by alkalies from its 

 solutions in acids and re-dissolved by excess. From neither of these 

 solutions is it thrown down by heat. It is dissolved by neutral salts, 

 and from this solution heat throws it down in an insoluble precipitate. 

 From its slightly alkaline solution in water it is thrown down by a 

 stream of carbonic acid, and may be re-dissolved by passing air or 



