238 THE BLOOD 



powdered glass or quartz particles of apparently the same size 

 are without effect. Coarser glass suspensions are necessary to 

 produce the effect. Some other factor in addition to surface must 

 come into play or kephalin particles must have a wrinkled surface. 



Birds' blood is peculiar in that, if drawn with due care to prevent 

 contact with the tissues it may be kept liquid for a long time. The 

 avian thigmocytes even under normal conditions will not adhere 

 to glass surfaces. The addition of the slightest trace of tissue 

 extract (kephalin) causes immediate and extensive clotting. 



This gel formation has been studied ultramicroscopically. The 

 first sign of clotting is the appearance of blood platelets or thigmo- 

 cytes, and from them as centres liquid crystals of fibrin crystallise 

 out, forming a feltwork of long elastic threads. 



Anti-coagulants. Certain substances by preventing the calcium 

 from acting on the thigmocytes, i.e. on pro-thrombin, prevent 

 clotting. One of these, called heparin by Howell, arises in the 

 liver. Other substances are really anti-thrombins. Hirudin, a 

 proteose prepared from the buccal glands of the medicinal leech 

 belongs to this class and is injected in blood-pressure and similar 

 experiments to prevent the formation of clots in the cannulae. 

 Certain salts act on the calcium. For example, oxalates, fluorides, 

 etc., form insoluble calcium salts, citrates form a double salt in 

 which Ca is not a free ion. Differing from all the above anti- 

 coagulants which act whether injected into the blood stream or 

 added to shed blood, are those which prevent clotting only when 

 injected. Snake venom in amounts as small as -00001 gram per 

 Kg. suffices. Commercial peptone (mixture of proteoses and 

 peptones) injected into the circulation in the proportion of 0-3 

 gram per kg. produces a non-coagulable blood for an hour or so. 

 This class of coagulant seems to act by stimulating the liver to 

 manufacture heparin. Peptonisation and the injection of venom 

 may cause some alteration in the state of the thigmocytes. This 

 has not been investigated. Of course, plasma or blood from 

 which the fibrinogen or fibrin has been removed will not clot. 



Coagulants. Extracts of organs rich in kephalin, e.g. thymus, 

 testes, lymph glands, produce intravascular clotting. 



Similarly, certain snake venoms which contain large quantities of 

 thrombin cause clotting of the blood in the vessels and rapid death. 



Components, (ii) Crystalloids. 



There is nothing more remarkable than the maintenance of 

 a fairly constant concentration of crystalloids in plasma, under 



