34 THE BLOOD. 



ing substance by oxidation and deoxidation, which alters its absorptive effect on 

 the light. 



Viewed in thin layers by transmitted light, venous blood appears green. It is, 

 therefore, dichroitic. 



2. Composition. The arterial blood, so far as is kno-«'n, is uniform in nature 

 throughout ; but in passing through the capillary vessels into the veins, whilst it 

 generally acquires the common characters of venous blood, it i.mdei'goes special 

 changes in its jiassage through particular organs, so that the blood of all veins is 

 not alike in quality. Thus the blood of the hepatic vems differs from that of the 

 portal vein, and both are in various respects different from what might be 

 regarded as the common venous blood, which is conveyed by the veins of the 

 limbs, and of the muscular and cutaneous parts of the body generally. Moreover, 

 Bernard has sho-mi that the blood of veins returning from secreting glands differs 

 according to the state of functional activity of the organs. Wliilst their function 

 is in abeyance the blood in their veins is dark, as usual, but when secretion is 

 active, the blood, which then also flows much more freely and abundantly, comes 

 through from the arteries to the veins with very little, if any, reduction of its 

 arterial brightness : it also retains nearly the whole of its separable oxygen. 



Compared with blood from a cutaneous vein, arterial blood is found to contain 

 a very little more water (about five parts in 1000) and to have a somewhat lower 

 specific gravity. The arterial plasma yields more fibrin and coagulates more 

 quickly ; the serum was said by Lehmann to contain less albumin and less fat, 

 but more extractive and a little more saline matter. Arterial blood yields more 

 oxygen gas, and less of both free and combined carbonic acid. 



Blood of the portal vein, compared with that of the jugular vein, was stated by 

 Lehmann to contain more water in proportion to solid matter, less fibrin and 

 albumin, more fat, extractive matter and salts. The pale corpuscles are vastly 

 more numerous than in venous blood generally. 



The Mood of the hepatic reins, according to Lehmann's statement, is richer in 

 both red and pale corpuscles, possibly from loss of water, and the proportion of 

 pale corpuscles to the red is increased. The hepatic venous blood, moreover, yields 

 sugar, derived from glycogen formed in the liver. 



The blood of the renal veins was stated by Bernard and Brown-Sequard not to 

 coagulate in the normal state of the kidney and its function : on trial, however, 

 we find that as regards coagulation it behaves like ordinary venous blood. 



COAGULATION OP THE BLOOD. 



In explaining the constitution of the plasma, ue have been obliged 

 so far to anticipate the account of the coagulation of the blood. The 

 following are the phenomena which usher in and which accompany 

 this remarkable change. Immediately after it is drawn the blood emits 

 a sort of exhalation, the " halitus," having a faint smell ; in about 

 three or four minutes a film appears on the surface, quickly spreading 

 from the circumference to the middle ; a minute or two later the part 

 of the blood in contact Avith the inside of the vessel becomes solid, 

 then speedily the whole mass ; so that in about eight or nine minutes 

 after being drawn, the blood is completely gelatinised. At about 

 iifteen or twenty minutes, or it may be much later, the jelly-like mass 

 begins to shrink away from the sides of the vessel, and the serum to 

 exude from it. The clot continues to contract, and the serum to escape 

 for several hours, the rapidity and degree of the contraction varying 

 exceedingly in diflFerent cases ; and, if the serum be poured off, more 

 will usually continue to drain slowly from the clot for two or three 

 days. 



Tlie nature of the change which takes place in the coagulation of the blood 

 has been already spoken of : it is essentially owing to the coagulation of the 



