CHAP. II.] THE BLOOD. 59 



It is more usual however to obtain serum by allowing blood (prefer- 

 ably arterial blood) to coagulate, when after some hours serum will 

 separate and can be decanted. 



The process of separation of serum is immensely facilitated and 

 the resulting serum is obtained most completely free from suspended 

 blood cells by subjecting recently coagulated blood to the action of a 

 centrifugal machine, such as is represented in the accompanying 

 figure. 



The blood as it flows from the blood-vessel is collected in stout 

 test-tubes provided with india-rubber stoppers. When the blood has 

 coagulated the tubes are fixed to the turn-table so that the stoppers 

 are directed centrally. The turn-table is then made to revolve with 

 great velocity for about half an hour, after which time the clot is 

 found to have retracted itself to the peripheral end of the tube, leaving 

 a large quantity of clear serum occupying the ends of the tube directed 

 towards the centre of the rotating disk. 



The centrifugal machine enables us to obtain in a short. time 

 considerable quantities of perfectly clear serum, which is thus obtained 

 before any putrefactive change can have affected its composition. 

 When serum has been merely decanted from the clot it is generally 

 more or less reddish from the presence of suspended corpuscles. From 

 such reddish serum, serum quite free from corpuscles can be obtained 

 by subjecting it to rotation in the centrifugal machine for about half 

 an hour. 



Description The serum which separates from the blood of a 



of physical healthy man, whilst fasting, is a liquid of a transparent 

 characters of n J , ' V1 ,. -, , , ' . ^ . . n r , , e 



serum. yellow colour like light sherry wine, varying in depth of 



colour but always perfectly clear. In the lower animals 

 the colour of the serum differs somewhat, being colourless in the 

 rabbit, amber coloured in the horse, of a very red amber tint in the ox, 

 and in the dog somewhat yellowish, nearly identical with that of man. 

 After a full meal the serum ceases to be transparent and becomes more 

 or less milky in appearance ; this phenomenon is usually described as 

 occurring only after an abundant fatty diet, but although seen to 

 greatest advantage after such a diet it constantly occurs after a full 

 meal of meat. 



The observations of Dr Andrew Buchanan 1 on this matter are of 

 great interest, and two of them are quoted as illustrating the above 

 statement : 



"A vigorous man of about 35 years of age, after fasting 19 hours, 

 had for dinner, twenty ounces of beef-steak, sixteen liquid ounces of brown 

 soup and eight ounces of bread. He was bled immediately before his 

 ifieal and three times after it, two ounces of blood being taken away each 

 time. The serum obtained from the first bleeding before the meal was 

 perfectly limpid; the serum from the second bleeding, three hours and 



1 Buchanan, "On. the white or opaque serum of blood," Proceedings of the Philo- 

 sophical Society of Glasgow, Vol. i. (18414), p. 226. 



