56 STRUCTUKE AND ECONOMY OF THE HORSE. 



by external temperature. It was not till comparatively a recent 

 date, that the blood has been considered to possess vitality, 

 which, however, is now generally acknowledged. The vitality 

 and fluidity of the blood is intimately associated ; in fact, its 

 coagulation, when removed from the body, constitutes its death. 

 The time in which this is taking jjlace is different in different 

 animals, and is influenced by various circumstances. In strong 

 animals, such as the horse, it is longer than in such weak animals 

 as the sheep : in the former, it is often as long as fifteen minutes ; 

 and, if the body be in a state of plethora, the vital power being 

 too highly developed, the death of the blood is much longer 

 resisted. In these cases coagulation is delayed, and, in conse- 

 quence, the red portion of the blood, being the heaviest, falls to 

 the bottom of the vessel, and the fibrin remains at the top, con- 

 stituting the buffy coat of inflammation. This separation, when 

 arising from the above cause, takes place long before the serum 

 is developed. 



The coagulation of the blood has been endeavoured to be 

 accounted for without success : it was held by some, that it was 

 produced by the cessation of its motion ; but it has been found 

 that, if stirred in a vessel, it will coagulate quicker than before. 

 It Avas thought that exposure to the atmosphere was the cause ; 

 but it has been known to coaiiulate in a vacuum, and likewise in 

 the body when a vein has been tied.* It was next conceived that 

 it was caused by the low temperature to which it is exposed; 

 but it has been ascertained, that it will coagulate quicker, if the 

 temperature is either higher or lower than natural ; but if so 

 low as to freeze the blood, it will not coagulate when afterwards 

 thawed. These experiments show, that the blood is analogous 

 to no other fluid, and that coa2;ulation cannot be owing to 

 physical causes, but can be explained only by reference to its 

 vitality. 



Although the blood will coagulate In the body if obstructed, 

 yet there is a considerable difference between this state, and its 

 coagulation out of the body. In the former instance coagulation 

 is longer occurring, new vessels are thx'own into its substance, 

 and it becomes organised. So, likewise, if a part be Avounded, 

 the divided vessels thi'ow out clots of blood, which adhere to the 

 surface of the wound ; the red particles become absorbed, the 

 glutinous fibrin organised, and the breach is thus gradually 

 restored. Thus we see how important it is that the blood should 



* Mr, James Turner has instituted and pviblished some singular experi- 

 ments on the blood vessels. He invented an instrrunent, by means of which 

 several inches of the carotid artery in a living horse was seized instanta- 

 neously, and the blood it contained suddenly imprisoned. It was then taken 

 from the body, and twenty-four hours after it was found that the blood was 

 in a florid and liquid state. 



