230 Dr. Buchanan an the White or Opaque Serum of the Blood. 



usual red colour on the surface, as also that drawn first after the meal 

 in the last case ; but in all the other instances it exhibited the same 

 pellucid fibrinous crust already described, although not dotted in the 

 same remarkable way. We can scarcely avoid the conclusion that this 

 pellucid crust is connected with finished digestion, when we reflect that 

 out of nine bleedings practised within eighteen hours after a very full 

 meal, this crust was observed on every occasion, if we except those in 

 which the blood was drawn within three hours and a quarter of the 

 period of taking the meal. 



These observations, the accuracy of which I have since had opportu- 

 nities of confirming, appear to me to leave no doubt as to the origin of 

 the white colour of the serum of the blood. When a healthy man is bled 

 fasting, his blood yields serum of a transparent yellow colour, like light 

 Sherry wine, varying in the depth of the yellow tint, but always perfectly 

 clear. In about half-an-hour after taking food, the serum becomes 

 turbid, the discolouration increases during several hours till it attains 

 its maximum, after which the serum becomes again gradually clearer, 

 till its limpidity is perfectly restored. The period at which the dis- 

 colouration is greatest, and the length of time during which it continues, 

 must depend mainly on the quantity of food taken, but also in some 

 degree on its quality, as some kinds of food are digested more readily 

 than others. It may however be stated, so far as the observations I 

 have made enable me to judge, that after a full meal of different kinds 

 of food, the discolouration is greatest about six or eight hours after the 

 repast, and that probably somewhat more than an equal period elapses 

 before the serum regains its limpidity. The differences of colour, which 

 are considerable, probably depend on the different substances digested : 

 and it is interesting in this point of view to remark, that the colour 

 varies in the successive bleedings after the same meal, as if the differ- 

 ent alimentary principles produced different kinds of discolouration, 

 and entered the blood-vessels at different periods. 



It may be inferred from the facts narrated above, that the food di- 

 gested in the stomach and bowels is introduced into the system, and 

 mingled with the blood in a crude or half assimilated state ; and that 

 it requires to undergo a second digestion within the blood-vessels be- 

 fore it is perfectly assimilated. It is a highly interesting inquiry by 

 what means this second digestion in the blood-vessels is effected. The 

 analogy of plants would indicate the lungs as being the principal agents, 

 for we find the crude sap brought by the sap-vessels to the leaves or 

 organs of respiration, converted by them into the succus proprius or 

 true blood of the plant. The respiratory act in man is not confined 

 to the lungs, but takes place in every part of the system to which the 

 absorbed oxygen is carried by the arterial blood : but it is a confir- 

 mation of the view just suggested, that at no time do we feel the want 

 of free air more severely than soon after a full meal. In all proba- 

 bility, however, the process of assimilation in the animal body is 



