THE BLOOD. 6? 



4. The addition of neutral salts in the proportion of 2 or 3 per cent 

 and upwards. When added in large proportion most of these saline 

 substances prevent coagulation altogether. Coagulation, however, ensues 

 on dilution with water. The time during which blood can be thus pre- 

 served in a liquid state and coagulated by the addition of water, is quite 

 indefinite. 



5. Imperfect aeration as in the blood of those who die by asphyxia. 



6. In inflammatory states of the system the blood coagulates more 

 slowly although more firmly. 



7. Coagulation is retarded by exclusion of the Uoodfrom the air, as by 

 pouring oil on the surface, etc. In vacuo, the blood coagulates quickly ; 

 but Lister thinks that the rapidity of the process is due to the bubbling 

 which ensues from the escape of gas, and to the blood being thus brought 

 more freely into contact with the containing vessel. Receiving blood 

 into a vessel, well smeared inside with oil, fat, or vaseline, is said also to 

 retard or prevent coagulation. 



8. The coagulation of the blood is prevented altogether by the addi- 

 tion of strong acids and caustic alkalies. 



9. It has been believed, and chiefly on the authority of Hunter, that 

 after certain modes of death the blood does not coagulate ; he enumerates 

 the death by lightning, over-exertion (as in animals hunted to death), 

 blows on the stomach, fits of anger. He says, <( I have seen instances of 

 them all." Doubtless he had done so ; but the results of such events are 

 not constant. The blood has been often observed coagulated in the 

 bodies of animals killed by lightning or an electric shock ; and Gulliver 

 has published instances in which he found clots in the hearts of hares 

 and stags hunted to death, and of cocks killed in fighting. 



10. The injection of peptones, or of various digestive ferments, e. g., 

 trypsin or pepsin, into the vessels of an animal appears to prevent or 

 stay coagulation of its blood if it be killed soon after. The secretion 

 of the mouth of the leech, and possibly the blood squeezed out of its body 

 when full, also prevents the clotting if added to blood. 



Cause of the fluidity of the blood within the living body. 



Very closely connected with the problem of the coagulation of the blood 

 is the question why does the blood remain liquid within the living body? 

 We have certain pathological and experimental facts, apparently opposed 

 to one another, which bear upon it, and these may be, for the sake of 

 clearness, classed under two heads: 



(1) Blood will coagulate within the living body under certain condi- 

 tionsfor example, on ligaturing an artery, whereby the inner and 

 middle coats are generally ruptured, a clot will form within it, or by 

 passing a needle through the coats of the vessel into the blood stream a 

 clot will gradually form upon it. Other foreign bodies, e.g., wire, thread, 

 etc., produce the" same effect. It is a well-known fact that small clots 

 are apt to form upon the roughened edges of the valves of the heart when 



